HOW  TO   USE 


THE 


MICROSCOPE 


BEING 


PRACTICAL  HINTS  ON  THE  SELECTION  AND 
USE  OF  THAT  INSTRUMENT, 


INTENDED  FOR  BEGHsTKEES. 


'      'B'sr    rTO 

:f 

EDITOR  OF  THE    "AMERICAN  JOURNAL  OP  MICROSCOPY.' 


SIXTH  EDITION. 

THOROUGHLY  REVISED  AND  GREATLY  ENLARGED, 

Illustrated  with  Eight  Plates  and  numerous  Figures  in  the  Text. 

NEW   YORK: 

THE  INDUSTRIAL  PUBLICATION  COMPANY. 

1890. 

COPYRIGHT   SECURED. 


P5- 


TO 

Mrs.   £IUB   < 

OF  HAVANA,  N.  Y., 

IN    MEMORY    OF    MANY    PLEASANT    HOURS    SPENT    OVER    THE    MICROSCOPE    WITH 
DEAR  ONES  WHO    ''ft^va^ONE^EFjbgE*' .f  njs  llTTLE    BOOK 

IS 


THE  AUTHOR. 


CONTENTS. 


DEDICATION.  -          - iii 

PREFACE.  -...----        vii 

INTRODUCTION.       .-  .  «-----xi 

THE  MICROSCOPE. 

What  it  Is ;  What  it  Does ;  Different  Kinds  of  Microscopes ;  Prin- 
ciples of  its  Construction,  and  Glossary  of  Terms ;  Essential 
Parts  of  the  Microscope ;  Names  of  the  Different  Parts.  -  15 

SIMPLE  MICROSCOPES. 

Hand  Magnifiers,  with  One,  Two,  and  Three  Lenses;  Doublets; 
Power  of  Two  or  More  Lenses  When  Used  Together;  Watch 
Makers'  Eye-Glasses—Single  Lenses  and  Doublets;  Engravers' 
Glasses ;  Linen  Provers ;  Stanhope  Lens ;  Stanhope  Collecting 
Microscope ;  Coddington  Lens ,  Achromatic  Doublets  and  Trip- 
lets; Twenty-five  Cent  Microscopes— Their  Construction,  and 
How  to  Make  Them ;  Penny  Microscopes,  to  Show  Eels  in  Paste 
and  Yinegar ;  Craig  Microscope ;  Novelty,  Globe,  etc.,  -  28 

DISSECTING  MICROSCOPES. 

Essentials  of  a  Good  Dissecting  Microscope ;  Cheap  Stands  for 
Simple  Microscopes,  Excelsior  Microscope;  Raspail's  Micro- 
scope ,  Compact  Dissecting  Microscope ;  Binocular  Dissecting 
Microscope,  ---------40 

COMPOUND  MICROSCOPES. 

Cheap  Foreign  Stands;  French  Vertical  Microscopes;  Conversion 
of  Vertical  Microscope  into  Collecting  Microscope;  the  Boss 
Model ;  The  Jackson  Model ;  The  Continental  Model ;  The  New 
American  Model  •  Cheap  American  Stands ;  The  Binocular  Mi- 
croscope; The  Binocular  Eye-Piece;  The  Inverted  Microscope ; 
Lithological  Microscopes;  The  Aquarium  Microscope;  Micro- 
scopes for  Special.Purposes ;  "Class"  Microscopes,  -  46 

OBJECTIVES. 

Defects  of  Common  Lenses;  Spherical  Aberration;  Chromatic 
Aberration ,  Use  of  Diaphragms ;  Corrected  Objectives ;  Defining 
Power ,  Achromatism .  Aberration  of  Form ;  Flatness  of  Field ; 
Angular  Aperture;  Penetrating  Power ;  Working  Distance;  Im- 
mersion and  "Homogeneous"  Lenses;  Lens  Systems;  Duplex 
Fronts ,  French  Triplets ;  Focal  Length  of  the  Numbers  used  to 
Designate  Objectives  by  Nachet,  Hartnack  and  Gundlaeh,  -  61 


M88211 


IV  CONTENTS. 


TESTING  OBJECTIVES. 

General  Rules ;  Accepted  Standards— Diatoms,  Ruled  Lines,  Arti- 
ficial Star;  Podura ;  Nobert's  Lines;  Holler's  Probe  Platte;  Table 
of  Diatoms  on  Holler's  Probe  Platte,  with  the  Number  of  Lines 
to  the  Inch  on  the  Several  Diatoms ;  Hethods  of  Testing  for  Flat- 
ness of  Field,  Penetration,  etc.,  84 

SELECTION  OF  A  MICKOSCOPE  FOB  PRACTICAL  PURPOSES. 

Must  be  Adapted  to  Requirements  and  Skill  of  User;  Microscopes 
for  Botany ;  For  Physicians ;  For  Students ;  Magnifying  Power 
Required ;  The  Stand ;  The  Stage ;  Mechanical  Stages ;  Revolving 
Stage ;  Stages  for  Special  Purposes ;  Diatom  Stage,  Safety  Stage, 
etc.;  Sub-Stage;  Mirror;  Body;  Draw-Tube;  Adjustments  for 
Focussing;  The  Diaphragm;  Objectives;  High  versus  Low 
Angles ;  Eye-Pieces,  - 95 

ACCESSORY  APPARATUS. 

Stage  Forceps;  Forceps  Carrier;  Object  Holder <  Plain  Slides; 
Concave  Slides ;  Watch-Glasses ;  Watch-Glass  Holder*  Animal- 
cule Cage;  Large  Zoophyte  Trough-  Small  Zoophyte  Trough; 
Walmsley's  Zoophyte  Trough;  The  Weber  Slide;  The  Cell- 
Trough  ;  The  Compressorium  >  Gravity  Compressorium ;  Grow- 
ing Slides ;  Frog  Plate ;  Table ;  Double  Nose-piece,  -  127 

ILLUMINATION— SOURCES  OF  LIGHT. 

Sun-Light;  Artificial  Light— Candles,  Gas,  Lamps,  Magnesium, 
Oxyhydrogen  Light ;  Parallel,  Convergent  and  Divergent  Rays,  143 

ILLUMINATION  OF  OPAQUE  OBJECTS. 

Diffused  Light ;  Bulls-Eye  Condenser ;  Side  Reflector ;  The  Lieber- 
kuhn ;  The  Parabolic  Reflector ;  Objectives  with  Tapered  Fronts ; 
Smith's  Vertical  Illuminator;  Tolles'  Vertical  Illuminator.  -  147 

ILLUMINATION  OF  TRANSPARENT  OBJECTS. 

Direct  and  Reflected  Light;  Axial  or  Central  Light;  Oblique 
Light ;  The  Achromatic  Condenser ;  The  Webster  Condenser,  and 
How  to  Use  it ;  Wenham's  Reflex  Illuminator,  and  How  to  Use 
it ;  The  Wenham  Prism ;  The  Hemispherical  Illuminator ;  The 
"  Half  Button  " ;  The  Woodward  Illuminator ;  Tolles'  Illumina- 
ting Traverse  Lens ;  The  Spot  Lens ;  The  Parabolic  Illumina- 
tor ;  Polarized  Light,  -  -  -  -  152 

How  TO  USE  THE  MICROSCOPE. 

General  Rules ;  Simple  Hand  Magnifiers ;  Compound  Microscopes ; 
Practical  Notes  on  Illumination;  White  Cloud  Illumination; 
Monochromatic  Light ;  Blue  Cell ;  Opaque  Objects ;  Hints  to  Be- 
ginners, -  -  164 

How  TO  USE  OBJECTIVES  OF  LARGE  APERTURE. 
Illumination ;  Collar-Correction  for  Cover-Glass,        -          -         -  171 


CONTENTS.  V 

CABE  or  THE  MICBOSCOPE.     . 

Should  be  Kept  Covered ;  Care  of  Objectives ;  Precautions  to  be 
Used  when  Corrosive  Vapors  and  Liquids  are  Employed;  To 
Protect  the  Objectives  from  Vapors  which  Corrode  Glass;  Clean- 
ing the  Objectives ;  Cleaning  the  Brass  Work,  -  -  175 

COLLECTING  OBJECTS. 

Where  to  Find  Objects ;  What  to  Look  for;  How  to  Capture  Them ; 
Nets;  Bottle-Holders ;  Spoons;  Collecting  Walking  Cane ;  Water 
Strainer ;  Wright's  Collecting  Bottle ;  Aquaria  for  Microscopic 
Objects;  Dipping  Tubes,  -.  -  177 

THE  PBEPABATION  AND  EXAMINATION  OF  OBJECTS. 

Cutting  Thin  Sections  of  Soft  Substances;  Valentine's  Knife;  Sec- 
tions of  Wood  and  Bone;  Improved  Section  Cutter;  Sections  of 
Rock ;  Knives ;  Scissors ;  Needles ;  Dissecting  Pans  and  Dishes ; 
Dissecting  Microscopes ;  Separation  of  Deposits  from  Liquids; 
Preparing  Whole  Insects;  Feet,  Eyes,  Tongues,  Wings,  etc.,  of 
Insects ;  Use  of  Chemical  Tests ;  Liquids  for  Moistening  Objects ; 
Refractive  Powers  of  Different  Liquids;  lod-Serum;  Artificial 
lod-Serum ;  Covers  for  Keeping  out  Dust ;  Errors  in  Microscopic 
Observations,  -  -  187 

PBESEBVATTVE  PBOCESSES. 

General  Principles ;  Preservative  Media— Canada  Balsam,  Solution 
of  Balsam,  Colophony,  Damar  Medium.  Glycerine,  Glycerine 
Jelly.  Hantzsch's  Fluid,  Glycerine  and  Gum,  Deane's  Gelatine, 
Alcohol,  Thwaite's  Fluid,  Beale's  Liquid,  Goadby's  Fluids,  Pa- 
cini's  Fluid,  Castor  Oil ;  General  Rules  for  Applying  Preservative 

Fluids,  -          -  198 

APPABATUS  FOB  MOUNTING  OBJECTS. 

Slides;  Covers;  Cells;  Turn-Tables—Plain,  Matthew's,  Kinne's, 
Cox's;    Cards  for  Making  Cells;    Hot-Plate;    Lamps;    Retort 
Stand;    Centering  Cards;  Mounting  Needles;  Cover  Forceps; 
Slide  Holder ;  Water  Bath ;  Simple  Form  of  Spring  Clip,       -        206 
CEMENTS  AND  VABNISHES. 

General  Rules  for  Using;  Gold  Size,  Black  Japan,  Brunswick 
Black,  Shellac.  Bell's  Cement,  Sealing  Wax  Varnish,  Colored 
Shellac,  Damar  Cement,  Marine  Glue,  Liquid  Glue,  Dextrine,  221 

MOUNTING  OBJECTS. 

Mounting  Transparent  Objects  Dry ;  Mounting  in  Balsam ;  Mount- 
ing in  Liquids;  Mounting  of  Whole  Insects ;  How  to  Get  Rid  of 
Air-Bubbles ;  Mounting  Opaque  Objects ;  Wooden  Cells ;  Leather 
Discs ;  Pierce's  Cell ;  Prof,  Smith's  Wax  Cell ;  Scott's  Metal  Cell ; 
Atwood's  Hard  Rubber  Cell ;  Dayton's  Shellac  Cell ;  Deep  Cells 
of  Litharge ;  Chester's  Method  of  Making  Cells  of  Tin  Foil,  -  224 


TO  CONTENTS. 

FINISHING  THE  SLIDES. 

Covering  with  Paper;  Finishing  with  Varnish;  Varnishing  for 
Preservation ;  Cleaning  and  Ornamenting  the  Slides,  -  -  232 

LABELING  THE  SLIDES. 

Importance  of  Properly  Labeling  Slides  and  Material;  the 
Author's  System ;  Use  of  the  Writing  Diamond ;  Designs  for 
Labels ;  Direction  of  the  Label,  -  -  233 

CABINETS. 

Various  Forms;  Backs;  Shallow  Drawers;  Direction  in  which 
the  Slides  should  Lie;  Facilities  for  Lifting  out  the  Slides,  -  234 

THE  MALTWOOD  FINDEE,        -  -          •          -          -  236 

MICROSCOPICAL  MISINTEEPEETATIONS. 

Sources  of  Fallacies;  Seeing  Objects  Pseudoscopically ;  Solid 
Cylinders  Appearing  as  Tubes;  Methods  of  Detecting  these 
Fallacies ;  Optical  Illusions ;  Circles  Appearing  as  Hexagons,  -  237 


PREFACE  TO  THIRD  EDITION. 

This  might  with  propriety  be  called  the  fourth  edition— the  main  idea 
of  the  work  having  been  embodied  in  a  pamphlet  under  the  same  title, 
published  in  1873.  but  long  since  forgotten.  The  successive  editions 
have  reached  their  present  form  by  accretion,  rather  than  by  develop- 
ment; like  Topsey.  the  book  has  "growed,"  rather  than  been  '  brought 
up.  and  just  as  that  young  lady  exhibited  numerous  traits  which  were 
inconsistent  with  a  proper  training,  so  this  book  shows  patch-work  and 
inequalities  which  do  not  add  either  to  its  value  or  its  attractiveness. 

Of  all  this  the  author  is  fully  conscious,  and  if  he  could  possibly  have 
secured  the  necessary  leisure  he  would  gladly  have  rewritten  the  entire 
volume.  But  pressing,  or  rather  imperative  calls  upon  his  time,  have 
prevented  this,  and  he  has,  therefore,  been  compelled  to  feel  satisfied 
with  such  general  revision  and  additions  as  were  necessary  to  bring  the 
work  up  to  the  present  state  of  our  knowledge. 

The  book  is  still  intended  for  beginners  and  has  changed  nothing  of 
its  elementary  character.  It  is  true  that  we  have  inserted  a  few  pages 
relating  to  the  higher  class  of  objectives,  and  the  accessories  used  with 
them,  but  those  who  do  not  possess  such  apparatus  can  easily  skip 
these  passages. 

That  it  is  suited  to  the  purpose  for  which  it  was  intended,  we  have 
evidence,  not  only  in  the  extended  sale  which  it  has  secured,  but  in 
the  fact  that  it  has  been  adopted  as  an  auxiliary  text-book  m  several 
of  our  schools  and  colleges. 

New  York,  January,  1881. 


PREFACE  TO  SECOND  EDITION. 

!  The  fact  that  an  unusually  large  edition  of  this  work  has  been  sold  in 
a  comparatively  short  period,  is,  to  the  author,  evidence  that  such  a 
work  was  needed,  and  that  the  present  volume  has,  to  a  certain  extent, 
supplied  the  want.  In  the  present  edition,  therefore,  he  has  endeavored 
to  introduce  several  important  improvements,  while  at  the  same  time 
the  elementary  character  of  the  work  remains  unaltered.  With  a  few 
very  slight  and  unimportant  exceptions,  the  entire  matter  of  the  former 
edition  has  been  incorporated  in  the  present,  and  in  addition  several 
important  subjects,  particularly  the  chapter  on  objectives,  have  toen 
greatly  enlarged. 


Till  PREFACE. 

Many  important  points  still  remain  untouched,  but  it  is  believed  that 
in  its  present  form  most  beginners  will  find  in  it  all  the  information 
that  they  may  require  upon  general  topics. 

As  the  want  of  all  illustrations  of  the  stands  of  different  makers,  and 
of  many  accessories,  has  been  urged  as  an  objection  to  the  first  edition, 
and  as  we  have  not  deemed  it  advisable  to  fully  supply  this  omission  in 
the  present  issue,  a  word  of  explanation  may  not  be  out  of  place.  One 
great  object  in  view  in  the  preparation  of  this  book  was  the  furnishing 
of  a  cheap  manual  for  those  who  cannot  afford  the  more  expensive  books 
of  Carpenter,  Beale,  Frey,  etc.  To  have  given  anything  like  a  fair  rep- 
resentation of  the  products  of  the  different  makers  of  this  country  and 
of  Europe,  would  have  nearly  doubled  the  size  and  price  of  the  volume. 
But  if  the  reader  will  examine  the  engravings  of  stands,  etc.,  in  the 
books  just  mentioned,  he  will  find  that,  even  in  the  best  of  them,  these 
illustrations  are  mere  reproductions  of  the  figures  found  in  the  descrip- 
tive lists  of  the  various  dealers.  As  new  editions  of  these  lists  are  being 
constantly  issued,  and  as  they  may  in  most  cases  be  obtained  without 
cost  from  those  that  publish  them,  we  have  thought  it  best  to  refer  our 
readers  to  these  catalogues  for  information  in  regard  to  the  construction 
of  the  instruments  of  different  makers.  For  the  addresses  of  the  promi- 
nent microscope  makers  of  this  country  and  Europe  we  refer  our 
readers  to  "The  Microscopist's  Annual." 

In  this,  as  in  the  previous  edition,  we  have  omitted  all  descriptions  of 
objects,  believing  that  the  proper  aim  of  a  book  on  the  microscope 
should  not  be  to  teach  the  general  principles  of  botany,  zoology  or 
histology,  but  simply  the  best  methods  of  using  the  microscope  in  the 
pursuit  of  these  studies.  The  proper  books  in  which  to  find  a  descrip- 
tion of  objects,  are  those  which  treat  of  that  department  of  science  which 
takes  cognizance  of  the  special  subject  under  consideration.  The 
present  volume  is  intended  merely  as  a  guide  to  the  best  general 
methods  of  using  the  microscope. 

It  has  been  a  source  of  great  satisfaction  to  the  author  to  be  assured 
by  those  whom  he  deems  good  authority,  that  this  little  book  has  done 
much  to  foster  the  use  of  the  microscope  in  this  country,  and  he  hopes 
that  the  present  improved  edition  will  tend  to  still  further  increase  the 
deep  interest  which  is  already  felt  in  an  instrument  which  has  done 
more  than  any  other  to  extend  our  knowledge  of  organic  nature. 

New  York,  August,  1877. 


PREFACE. 


The  Microscope  and  its  applications  in  the  Arts,  and  in  general  science, 
having  deservedly  occupied  a  prominent  place  iu  the  pages  of  TEE  TECH- 
NOLOGIST, OE  IXDUSTBIAL  MONTHLY,  a  very  large  number  of  enquiries  in 
regard  to  the  best  methods  of  using  and  applying  this  useful  instrument 
Lave  been  directed  to  us.  It  would  have  been  easy  to  answer  these  en- 
quiries by  a  reference  to  some  one  of  the  many  treatises  that  have  been 
1  ublished  on  this  subject,  but  as  most  of  these  works  are  expensive,  and 
as  many  of  our  correspondents  desire  an  answer  in  a  more  concise  and 
simple  shape,  we  have  endeavored  to  give,  in  cheap  and  compact  form,  the 
information  that  is  most  usually  demanded. 

It  is  an  unfortunate  fact  that  while  the  microscope  is  daily  growing  in 
favor  with  those  who  know  anything  of  its  achievements,  the  operations 
of  certain  parties,  too  well  known  to  the  public,  have  brought  a  certain 
degree  of  suspicion  upon  all  attempts  to  popularize  this  most  valuable 
instrument.  Microscopes,  varying  in  price  from  twenty-five  cents  to  two 
dollars  and  a  half  have  been  offered  for  sale,  and  the  claim  made  for  them 
that  they  are  capable  of  showing  clearly  the  structure  of  the  more  min- 
ute tissues,  and  that  they  may  be  used  to  advantage  by  physicians  and 
naturalists.  To  the  young  student  whose  means  are  limited,  and  to  the 
country  practitioner,  whose  ability  to  supply  himself  with  needed  books 
and  instruments  often  falls  far  short  of  his  desires,  the  offer  of  a  service- 
able microscope  for  a  couple  of  dollars  is  a  great  temptation,  and  when 
the  instrument  in  question  is  endorsed  by  a  long  list  of  clergymen,  law- 
yers, and  even  editors,  this  temptation  becomes  irresistible.  And  if  the 
purchaser  should  happen  to  be  unfamiliar  with  really  good  microscopes, 
and  unable  to  discriminate  between  a  clear  and  accurately  denned  view 
of  any  object  and  one  that  is  distorted  and  incorrect,  he  may  be  led  to  use 
it,  and  so  fall  into  the  most  serious  mistakes.  That  this,  unfortunately, 
does  happen  too  often  must  be  well  known  to  all  who  are  familiar  with  the 
subject,  and  it  is  within  our  own  knowledge  that  the  most  worthless  cari- 


X  PREFACE. 

cature  of  a  microscope  has  been  purchased  and  used  under  guch  circum- 
stances. 

We  indulge  a  faint  hope  that  the  information  conveyed  in  the  following 
pages  will  enable  the  inexperienced  reader  to  avoid  these  mistakes,  and  to 
assign  a  proper  value  to  the  certificates  of  clergymen  and  editors  who 
vouch  for  the  excellence  of  articles  concerning  whose  properties  and  uses 
they  are  profoundly  ignorant.  These  two  classes  we  single  out  for  repro- 
bation, because— in  this  respect,  at  least — they  seem  to  be  sinners  above 
all  other  men. 

As  stated  in  the  title  page,  it  is  intended  for  beginners,  and  not  for 
beginners  in  the  use  of  the  microscope  only,  b*ut  for  those  who  have  had 
little  or  no  experience  in  the  use  of  instruments  of  any  kind.  Hence  the 
directions  that  are  given  are  of  the  very  simplest  kind,  arid  all  theoretical 
explanations  have  been  avoided,  for  the  reason  that  any  person  that  is 
desirous  of  studying  the  optical  principles  upon  which  the  microscope  is 
constructed  will  find  in  the  ordinary  text  books  on  natural  philosophy  all 
the  information  he  may  want.  Our  object  has  been  solely  to  impart  such 
information  as  will  enable  the  reader  to  make  a  beginning  in  the.  practice 
of  microscopy,  hoping  that  the  start  thus  given  will  lead  him  to  proceed 
with  his  studies,  and  ultimately  acquire  that  knowledge,  skill  and  dexter- 
ity which  will  enable  him  to  avail  himself  of  the  extraordinary  powers  and 
advantages  which  the  use  of  this  instrument  confers,  both  in  scientific 
pursuits  and  in  everyday  life.  Above  all  things,  therefore,  we  have  en- 
deavored to  be  accurate  in  our  statements  and  judicious  in  our  directions, 
and  the  reader  is  assured  that  no  processes  or  methods  are  given  which 
we  ourselves  have  not  frequently  and  successfully  put  in  practice. 


JOHN  PHIN. 
New  Yor1ct  January,  1875. 


INTRODUCTION. 

Thousands  of  microscopes  throughout  the  country  are  at  the  pres- 
ent day  lying  idle,  simply  because  their  owners  do  not  know  how  to 
use  them.  If  properly  employed  they  might  be  made  to  afford  an 
incalculable  amount  of  instruction  and  amusement;  but,  as  it  is,  they 
are  a  drag  upon  the  popularization  of  science,  because  they  convey 
the  Idea  that  the  microscope  is  a  difficult  instrument  to  use,  and 
that  it  is  not  of  much  account  after  we  have  learned  to  use  it.  The 
owners  of  these  microscopes  have  examined  all  the  mounted  objects 
at  their  command,  the  entire  number  of  which  probably  does  not 
exceed  two  or  three  dozen,  and  they  have  no  information  as  to  the 
best  methods  of  preparing  common  objects  for  examination  or  pre- 
servation. Even  the  objects  that  they  possess  have  never  been  ex- 
plained to  them,  and  are  merely  pretty  toys.  The  fly's  eye  is  inter- 
esting because  it  looks  like  a  piece  of  nettiug,  and  the  butterfly's 
wing  is  attractive  because  it  is  probably  a  little  more  brilliant  than 
the  most  brilliant  silk  dress,  but  neither  of  these  objects  interests  of 
itself  and  because  of  its  beautiful  structure. 

Moreover  it  often  happens  that  an  instrument  which,  when  first 
purchased,  was  of  very  fair  quality,  has,  through  ignorance  and 
carelessness,  become  so  soiled  and  dimmed  that  it  no  longer  serves 
the  purpose  intended.  On  more  than  one  occasion  have  we  seen  a 
fine  microscope  leave  the  dealer's  hands  in  excellent  order,  and  re- 
turn in  a  week  entirely  unfit  for  use.  Microscopes  in  this  condition, 
instead  of  being  a  source  of  instruction  and  pleasure,  are  an  eyesore 
and  an  occasion  of  annoyance.  They  continually  serve  as  reminders 
of  awkwardness  and  failure,  of  wasted  time  and  ill-spent  money. 
And  yet  with  proper  instruction  and  a  due  amount  of  care  all  this 
might  have  been  avoided. 

It  is  also  a  fact  to  be  regretted  that  heretofore  the  microscope  has 
not  been  extensively  employed  in  the  arts,  and  in  everyday  life, 
simply  because  practical  men  have  not  been  taught  how  to  use  it, 
and  consequently  have  been  unable  to  avail  themselves  of  the  advan- 
tages which  it  offers;  but  if  carefully  and  judiciously  selected,  and 
properly  handled,  it  is  capable  of  affording  an  amount  and  kind  of 
assistance  which  cannot  be  safely  neglected.  It  may  be  made  to  aid 
in  the  examination  of  raw  materials,  and  of  the  finer  kinds  of  work ; 
it  will  enable  us  to  measure  spaces  which  would  otherwise  be  inap- 
preciable, and  this,  in  an  age  when  even  in  ordinary  machine  shops 
the  thousandth  part  of  an  inch  is  frequently  an  important  quantity, 
renders  it  indispensable  to  the  careful  and  skillful  mechanic;  on  the 


Xll  INTRODUCTION. 

farm  it  will  enable  the  agriculturist  to  examine  closely  and  minutely 
the  various  noxious  insects  and  forms  of  fungi  and  blight,  and  thus 
aid  him  in  identifying  them  and  applying  the  proper  remedy;  and 
in  the  examination  of  minute  seeds,  such  as  timothy,  clover,  etc.,  it 
will  prove  a  very  valuable  assistant,  enabling  him  to  detect  any  in- 
feriority in  the  quality,  or  any  impurity  or  adulteration.  Frequently 
the  agricultural  seeds  offered  in  market  contain  minute  seeds  of  of- 
fensive weeds,  many  of  which  are  so  small  that  they  are  not  easily  dis- 
covered by  the  naked  eye. 

Every  farmer  and  mechanic  knows  the  value  of  a  good  pair  of 
eyes,  and  he  also  knows  that  an  agent  which  doubles  or  trebles  our 
power  in  any  given  direction  at  once  confers  upon  us  in  that  respect 
a  superiority  over  our  fellows.  Very  few  men  are  twice  as  strong  as 
their  comrades;  still  fewer  have  three  times  the  strength  of  ordinary 
men,  and  it  may  be  safely  affirmed  that  no  man  possesses  the  power 
of  ten  ordinary  men.  But  a  microscope  of  very  ordinary  capacity  at 
once  multiplies  our  powers  of  sight  by  ten,  twenty,  or  even  a  hun- 
dred times,  while  those  of  the  better  class  enable  us  to  see  things 
with  a  keenness  and  clearness  which,  when  compared  with  that  af- 
forded by  the  naked  eye,  is  as  more  than  a  thousand  to  one. 

There  are  four  distinct  and  important  directions  in  which  a  mi- 
croscope may  be  made  to  serve  us:  1.  It  is  capable  of  affording  the 
most  refined  and  elevating  kind  of  pleasure  by  the  exhibition  of  ob- 
jects of  extreme  beauty  and  interest.  There  are  few  more  splendid 
sights  than  the  gorgeous  colors  displayed  by.some  objects  when 
viewed  by  polarized  light,  and  even  the  tints  of  certain  minerals, 
and  the  brilliant  scales  of  certain  insects,  when  viewed  as  opaque 
objects,  by  means  of  a  good  condenser,  surpass  anything  that  is 
familiar  to  us  in  our  ordinary  experience.  On  the  other  hand  the 
exquisite  beauty  of  form  which  is  characteristic  of  most  of  the  ob- 
jects with  which  the  rnicroscopist  concerns  himself  can  be  fully  ap- 
preciated only  by  those  who  have  seen  them.  As  a  source  of  inno- 
cent amusement  and  pleasure,  therefore,  the  microscope  has  few  or 
no  equals;  for  it  may  be  safely  affirmed  that  a  five-dollar  instrument 
is  capable  of  affording  gratification  of  greater  variety  and  intensity, 
and  of  longer  continuance,  than  that  yielded  by  anything  else  of 
the  same  cost.  This  arises  chiefly  from  the  fact  that  most  other  in- 
struments, when  once  exhibited,  with  their  slides  or  fixtures,  lose 
their  freshness  and  interest,  and  become  old.  While  for  the  micro- 
scope, a  few  fibres  of  wool  from  the  carpet,  a  few  grains  of  sand  from 
the  sea-shore,  or  a  handful  of  wild  flowers  from  the  field,  yield  ob- 
jects of  surpassing  beauty.  Everything  in  nature  and  in  art  may  bo 


INTBODTTCTION.  XU1 

subjected  to  inspection  by  it,  and  will  tlien  disclose  new  beauties; 
;ind  fresh  sources  of  knowledge.  Under  it  the  point  of  the  finest 
cambric  needle  looks  like  a  crow-bar,  grooved  and  seamed  with 
scratches;  the  eye  of  the  fly  is  seen  to  consist  of  thousands  of  eyes; 
aud  the  dust  on  the  butterfly's  wing  appears  to  be  what  it  really  is, 
scales  laid  on  with  all  the  regularity  of  shingles  or  slates  on  a  house; 
while  to  prepare  and  examine  these  simple  objects  requires  no  great 
skill  and  no  elaborate  apparatus. 

2.  As  a  means  of  imparting  instruction  to  the  young,  the  micros- 
cope has  now  become  indispensable.     The  changes  which  of  late 
years  have  taken  place  in  the  views  held  by  our  ablest  men  in  regard 
to   the  best  education  are  too  well  known  to  need  even  mention. 
No  education  that  dots  not  include  a  knowledge  of  natural  science 
is  now  regarded  as  complete,  and  there  is  a  very  wide  range  of  the 
most  essential  and  practical  knowledge  that  can  be  reached  only 
through  the  microscope.     Thus,  when  we  look  at  a  leaf  with  the 
naked  eye,  we  see  but  a  green  mass  of  matter,  possessing  a  certain 
beautiful  form,  it  is  true,  but  disclosing  none  of  those  organs  which 
render  it  more  complicated  and  wonderful  than  anything  ever  pro- 
duced by  our  most  skilful  mechanics.     Looked  at  by  the  micro- 
scope, however,  this  same  leaf  is  found  to  be  made  up  of  innumer- 
able parts,  each  one  of  which  is  highly  complex  and  beautiful;  it  is 
furnished  with  mouths  for  breathing,  with  cells   for   storing,  digest- 
ing and  assimilating  nutriment,  and  with  ribs  for  strengthening  its 
structure;  and  all  this,  which  is  perfectly  invisible  to  the  unassisted 
vision,  becomes  distinct  and  obvious  when  we  call  to  our  aid  a 
microscope  of  even  moderate  power.     It  is  true  th-dt  much  of  this 
may  be  taught  by  means  of  books,  engravings  and  verbal  descrip- 
tions, but  every  one  knows  that  for  distinctness  and  impressiveneis 
the  very  best  engravings  fall  far  short  of  a  view  of  the  real  object. 

3.  As  an  instrument  of  research,  the  microscope  now  occupies  a 
position  which  is  second  to  none.     There  is  hardly  any  department 
of  science  in  which  a  student  can  hope  to  reach  eminence  without  a 
familiarity  with  the  microscope.     Botany  and  Zoology  have  been 
developed  almost  wholly  by  its  aid,  aud  so  necessary  is  it  in   the 
study  of  these  sciences,  that  Schleiden,  one  of  the  most  successful 
of  investigators  says  of  it:    "  He  who  expects  to  become  a  botanist 
or  a  zoologist  without  using  th%  microscope,  is,  to  say  the  least  of 
him,  as  great  a  fool  as  he  who  wishes  to  study  the  heavens  without  a 
telescope."    In  chemistry  its  services  have  been  very  important,  and 
in  geology  and  mineralogy  it  has  opened  .up  new  fields  of  research 
which  almost  promise  to  revolutionize  these  sciences.     Medicine  has 


XIV  INTRODUCTION. 

long  acknowledged  the  microscope  as  one  of  its  most  efficient  as~ 
sistants,  and  in  the  practice  of  the  best  physicians  it  is  regarded  as 
an  indispensable  means  of  diagnosis  in  £ome  diseases. 

4.  As  an  assistant  in  the  arts.  Its  importance  in  this  department 
is  but  just  beginning  to  be  recognized,  and  in  a  former  paragraph 
we  have  endeavored  to  point  out  a  few  of  the  subjects  to  which  it 
may  be  applied  with  good  hopes  of  success. 

These  important  and  obvious  advantages  are  not  difficult  to  secure, 
provided  we  avoid  two  mistakes  which  are  very  commonly  made  by 
beginners.  One  of  these  consists  in  supposing  that  it  is  only  by 
means  of  very  expensive  and  complicated  instruments  that  anything 
of  value  can  be  accomplished  in  microscopy.  Now  while  it  is  cer- 
tain that,  in  some  departments  of  study,  none  but  the  very  best 
microscopes  are  of  any  value  at  all,  it  is  equally  certain  that  a  very 
wide  range  of  study  and  of  practical  work  can  be  thoroughly  culti- 
vated by  means  of  apparatus  of  very  moderate  cost,  and  of  great 
simplicity  of  construction.  The  great  discoveries  of  Ehrenberg, 
which  opened  up  entire  new  fields  of  research  and  of  thought,  were 
made  with  a  microscope  which  at  the  present  day  would  not  com- 
mand $25.  Indeed  some  of  the  French  instruments  that  are  sold 
for  $15  will  show  a  very  large  proportion  of  the  objects  that  are 
figured  in  his  earlier  works.  Most  of  the  great  anatomical  ar;d 
botanical  discoveries  were  made  with  simple  microscopes  of  no  great 
power,  and  it  is  not  many  years  since  one  of  the  most  successful 
workers  in  the  field  of  botany  gave  it  as  his  opinion  fiat  a  power  of 
300  diameters  is  capable  of  showing  everything  that  is  of  impor- 
tance in  this  science. 

The  other  error  is  of  precisely  the  opposite  kind.  It  is  not  at  all 
unusual  to  meet  persons  who  seem  to  think  that  all  that  is  necessary 
in  order  to  become  a  microscopist  is  to  buy  a  microscope  and  place 
objects  under  it!  Such  people  always  entertain  an  exaggerated  idea 
of  the  power  of  the  microscope  as  an  instrument  of  research.  For 
example,  they  think  that  in  order  to  detect  adulteration  all  that  is 
jpecessary  is  to  place  a  sample  under  the  microscope,  when  all  im- 
purities will  at  once  stand  out  conspicuously!  To  their  imagination 
every  blood  corpuscle  is  clearly  marked  with  the  name  of  the  animal 
from  which  it  was  obtained!  * 

Truth  lies  between  these  extreme's.  No  progress  can  be  made 
without  steady  application  and  persistent  labor,  but  any  person  of 
fair  average  ability  and  a  moderate  degree  of  perseverance  can  soon 
learn  to  follow  the  beaten  track  at  least,  if  not  to  branch  out  into 
original  research. 


THE  SELECTION  AND  USE 

OF 

THE    MICROSCOPIC. 


What  is  a  Microscope  'I— The  microscope  is  an  instru- 
ment which  enables  us  to  see  either  very  minute  objects  or 
very  minute  parts  of  large  objects.  It  is  a  very  popular  idea 
that  the  name  microscope  is  applicable  only  to  complex  instru- 
ments of  considerable  power;  but  this  is  clearly  wrong.  A  ten 
cent  magnifying  glass  has  as  good  a  right  to  the  name  micro- 
scope as  has  a  complicated  binocular  instrument  with  all  the 
latest  improvements.  By  common  consent,  however,  the  small 
hand  instruments,  without  stands,  are  generally  called  magni- 
fiers. An  attempt  has  been  made  to  introduce  the  foreign  word 
loupe  as  an  equivalent  of  magnifier.  The  word  loupe  is,  how- 
ever, superfluous,  and  is  used  only  by  ostentatious  pedants,  and 
by  foreigners  who  are  ignorant  of  English. 

What  the  Microscope  Does.— It  is  well  known  that 
the  further  off  any  object  is,  the  less  it  appears.  A  house  at  a 
distance  appears  less  than  a  man  who  is  close  by,  and  the  dis- 
tinctness with  which  an  object  is  seen  depends  largely  upon  its 
apparent  size.  Thus,  at  a  distance,  a  house  not  only  appears 
very  small,  but  the  windows  cannot  be  distinguished  from  the 
rest  of  the  building.  As  we  draw  nearer  it  becomes  apparently 
larger,  and  the  different  parts  become  more  distinct.  First  the 
windows  are  seen  clearly,  then  the  individual  panes  of  glass, 
then  the  bricks,  and  finally  the  grains  of  the  material  of  which 
the  bricks  are  made.  When,  however,  we  approach  too  closely 
we  again  find  it  impossible  to  see  distinctly,  as  may  easily  be 


16  SELECTION    AND    tJSE 

proved  by  a  very  simple  experiment.  Place  some  fine  print, 
such,  for  example,  as  the  present  page,  at  a  distance  of  six  feet 
from  the  eye,  and  gradually  move  closer  to  it.  At  six  feet  the 
letters  will  be  indistinguishable;  at  two  feet  they  will  be  quite 
distinct;  at  one  foot  still  more  distinct;  at  three  inches  they 
will  be  quite  blurred.  There  is,  therefore,  a  limit  to  the  degree 
of  closeness  with  which  we  can  approach  any  object  for  the 
pTiypc>3e  of,  e^ta^in^ng  |t,  and  the  object  of  a  microscope  is  to 
eriab^us^ttCget  cclbste  io  it,  as  it  were,  without  blurring  our 
view.  If,  without  changing  the  distance  of  the  eye  from  the 
jp^^ri^^hA<e<etin^ljes)(twecjnvtrtoduce  between  the  two  a  lens  of  one 
dricH  fo&ivancl  feKmg^it  "iftio  proper  position,  we  will  find  that 
the  indistinctness  formerly  complained  of  disappears,  and  the 
object  is  now  not  only  seen  clearly,  but  appears  very  much 
magnified.  That  objects  appear  large  in  proportion  to  their 
nearness  to  the  eye  may  be  thus  shown:  Take  two  slips  of 
paper  printed  with  type  of  the  same  size  .(two  clippings  from  a 
newspaper,  answer  well)  and  place  one  at  a  distance  of  ten 
inches  from  the  eye  and  the  other  at  a  distance  of  five  inches — 
the  edge  of  the  upper  slip  being  placed  so  as  to  lie  about  the 
middle  of  the  lower  one.  In  this  way  we  can  readily  compare 
the  apparent  sizes  of  the  type  on  the  two  slips,  and  one  will  be 
found  to  appear  just  twice  as  large  as  the  other,  though,  of 
course,  we  have  the  evidence  of  our  senses  to  prove  that  they 
are  precisely  of  the  same  size.  Moreover,  as  the  usual  distance 
for  distinct  vision  is  about  ten  inches,  in  persons  of  middle 
age,  it  will  be  found  that  a  lens  which  enables  us  to  view  any 
object  clearly  and  distinctly  from  a  distance  of  one  inch,  will 
enable  us  to  see  it  just  ten  times  larger  and  ten  times  more  dis- 
tinctly than  we  could  do  when  looking  at  it  from  a  distance  of 
ten  inches.  A  consideration  of  these  facts  led  the  late  Dr. 
Goring  to  propose  the  name  engiscope  as  a  substitute  for  the 
word  microscope — the  word  engiscope  signifying  to  see  things 
at  a  very  short  distance. 

The  facts  which  we  have  just  detailed  must,  however,  be  re- 
garded as  illustrations,  rather  than  explanations  of  the  action  of 
the  microscope.  It  is  evident  that  the  power  of  a  lens  to  in- 
crease the  distinctness  with  which  any  object  is  seen,  depends 
not  only  upon  the  action  of  the  lens  upon  the  rays  of  light,  but 
upon  the  influence  which  such  modified  light  exerts  upon  the 


0?  THE   MlCKOSfcOPE.  17 

organs  of  vision.  Now,  the  eye,  considered  merely  as  an  optical 
instrument,  is  in  reality  a  small  camera  obscura  in  which  the 
cornea,  crystalline  lens,  and  other  transparent  portions,  combine 
to  throw  upon  the  retina  an  image  of  external  objects.  That 
the  transparent  portions  of  the  eye  do  in  fact  act  as  a  lens,  and 
throw  a  real  image  upon  the  retina  or  posterior  portion  of  the 
eye,  is  easily  shown  by  taking  the  fresh  eye  of  an  ox  and  grad- 
ually shaving  off  the  coating  at  the  back  until  it  becomes 
transparent.  If  the  eye,  so  prepared,  be  then  held  towards 
a  window  or  any  very  bright  object,  a  distinct  but  inverted 
image  of  the  windovf  or  other  object  will  be  seen  on  the  coat 
of  the  eye. 

The  action  of  the  eye  in  this  case  is  the  same  as  that  of  a 
lens,  and  the  general  mode  of  action  of  lenses  under  such  cir- 
cumstances may  be  easily  illustrated  by  means  of  a  common 
hand  magnifier  or  even  a  spectacle  glass.  If  the  reader  will 
hold  before  a  -window,  at  a  distance  of,  say,  six  feet,  a  sheet  of 
white  paper,  and  will  place  a  magnifier  in  front  of  the  paper, 
then  by  properly  adjusting  the  distance  between  the  magnifier 
and  the  paper,  a  picture  of  the  window  will  be  thrown  on  the 
latter.  If  the  magnifier  and  paper  be  now  removed  to  a  dis- 
tance of  twelve  feet  from  the  window,  the  picture  of  the  latter 
will  be  only  half  as  large  as  it  was  in  the  first  place,  and  it  will 
also  be  found  that  the  distance  of  the  lens  from  the  paper  will 
have  to  be  readjusted  and  made  less. 

That  the  eye  possesses  this  power  of  adjustment  we  are  all 
conscious,  for  we  feel  that  if,  when  the  eye  is  adjusted  for  the 
distinct  vision  of  distant  objects,  we  suddenly  look  at  those 
which  are  near,  the  condition  of  the  eye  requires  to  be  changed 
before  a  distinct  view  can  be  had,  and  to  make  this  change 
requires  an  effort  of  which  we  are  perfectly  conscious. 

When  a  lens  is  held  in  front  of  a  sheet  of  paper,  so  as  to 
throw  on  the  latter  a  distinct  image  of  the  objects  in  front  of 
it,  the  distance  between  the  paper  and  the  leus  is  called  the 
focal  distance  or  focal  length  of  the  latter.  This,  as  we  have 
just  seen,  varies  with  the  distance  of  t';e  object  which  gives  the 
image.  In  order,  therefore,  to  secure  a  standard  in  this 
respect  the  object  selected  is  always  one  whose  distance  is  so 
great  that  it  may  be  practically  regarded  as  infinite. 


18  SELECTION  AND    USE 

When  we  examine  an  object,  first  at  a  distance,  and  then 
close  at  hand,  we  see  it  through  the  medium  of  two  different 
sets  of  rays,  those  in  the  latter  case  entering  the  eye  in  such  a 
direction  that  the  image  thrown  on  the  retina  is  larger  than  the 
image  produced  when  the  object  is  more  distant.  The  lens  acts, 
however,  by  bending  the  rays  so  that  the  same  set,  which,  if 
allowed  to  pursue  their  natural  direction  would  not  produce  a 
distinct  image,  are  caused  to  enter  the  eye  in  such  a  direction  that 
the  image  is  large  and  clear.  The  manner  in  which  the  lens  acts 
to  produce  these  effects  is  not  difficult  to  understand.  It  is  true 
that  the  ultimate  causes  which  produce  these  phenomena  are 
beyond  our  knowledge,  but  in  this  respect  the  ablest  philos- 
opher has  very  little  advantage  over  the  veriest  tyro.  It  may 
be  difficult  also  for  the  general  reader  to  follow  the  mathe- 
matical demonstrations  of  the  action  of  lenses.  There  are, 
however,  a  few  simple  facts  which  are  easily  understood,  or  at 
least  demonstrated  and  accepted  as  facts,  and  which,  when 
clearly  and  firmly  grasped  by  the  mind,  render  the  construction 
of  the  microscope  comparatively  easy  of  comprehension. 

There  are  two  ways  in  which  the  subject  may  be  studied. 
We  may  examine  the  facts  experimentally,  by  using  lenses 
and  actual  eyes  in  the  way  we  have  described,  or  we  may  fol- 
low the  course  of  the  rays  as  laid  down  in  any  good  book  on 
optics.  A  combination  of  both  methods  will  of  course  give 
the  clearest  views  on  the  subject,  and  we  would  therefore  ad- 
vise the  reader  to  provide  himself  with  a  few  lenses  of  various 
degrees  of  curvature,  and  consequently  of  various  magnifying 
powers,  and  test  all  the  statements  made  in  the  text.  He  will 
thus  acquire  such  a  practical  knowledge  of  the  action  of  lenses 
as  can  be  obtained  in  no  other  way.  For  this  purpose  the 
cheapest  lenses  are  good  enough.  One  or  two  cheap  magni- 
fiers and  a  few  glasses  from  old  spectacles  will  serve  every  pur- 
pose. The  simplest  methods  of  arranging  such  lenses  will  be 
found  in  a  note  on  a  subsequent  page,  and  although  very  ac- 
curately made  tools  are  required  for  the  construction  of  ser- 
viceable optical  instruments,  it  will  be  found  that  a  very 
large  number  of  simple  but  valuable  experiments  may  be 
worked  out  with  the  aid  of  a  few  wooden  rollers  and  a  little 
paper  and  paste. 


OP  t££  mcnoscrii'ii  19 

"While  the  magnifying  power  of  lenses  depends  upon  their 
focal  length,  this  in  turn  depends  upon  the  material  of  which 
the  lens  is  made,  and  also  upon  the  curvature  given  to  its  sur- 
faces. Lenses  of  precisely  the  same  form,  and  made  respect- 
ively of  diamond,  flint  glass,  crown  glass  and  Canada  balsam 
would  possess  different  magnifying  powers;  the  diamond  mag- 
nifying most,  the  flint  glass  next,  crown  glass  next,  and  Canada 
balsam  least  of  all.  On  the  other  hand,  of  two  lenses  composed 
of  the  same  material,  that  which  has  the  sharpest  curvature  to 
its  surfaces  will  magnify  most  Now,  on  reflection,  it  will  be 
evident  to  even  the  least  mathematical  mind  that  lenses  which 
have  very  sharp  or  quick  curves  must  of  necessity  be  small. 
Suppose  the  curve  which  bounds  the  figure  of  a  lens  has  a 
radius  of  half  an  inch,  it  is  evident  that  the  largest  lens  which 
could  be  made  with  this  curve  would  be  one  inch  in  diameter, 
and  then  it  would  be  a  perfect  sphere.  Most  lenses,  however, 
resemble  thin  slices  off  the  spheres,  or  in  some  cases  two  such 
slices  joined  together,  so  that  the  diameter  of  the  lens  is  in 
general  greatly  less  than  the  radius  of  the  curves  which  form 
its  surface.  Therefore,  we  see  that  all  lenses  of  high  power  are 
of  necessity  small,  and  when  lenses  are  required  of  very  high 
power  they  become  so  minute  as  to  be  handled  only  with  great 
difficulty.  Indeed,  before  the  modern  improvements  in  the 
microscope,  many  of  the  lenses  used  by  scientific  men  were 
nothing  more  than  little  globules  of  glass,  brought  to  a  round 
form  by  fusion. 

We  have  made  this  lengthened  explanation  of  a  very  simple 
matter  because  we  have  found  amongst  beginners  in  micro- 
scopy a  very  general  idea  that  large  lenses  are  the  most  power- 
ful "Send  me  one  of  your  largest  and  most  powerful  mag- 
nifiers," is  an  order  with  which  every  optician  is  familiar,  and 
yet  such  an  order  contains  a  contradiction  in  terms.  A  lens 
cannot  possibly  be  large  and  magnify  greatly  at  the  same  time. 

The  Different  Kinds  of  Microscopes.— Microscopes 
are  divided  into  two  classes — simple  and  compound — the  dif- 
ference between  them  being  purely  optical,  and  not  mechanical; 
for  a  simple  microscope  may  be  very  complex  and  expensive, 
while,  on  the  other  hand,  a  microscope  may  be  compound  and 


20  SELECTION   AND   tJSE 

yet  contain  very  few  parts.  Thus  the  little  vertical  French 
microscopes,  which  cost  only  $2,50,  are  compound,  although 
very  simple  in  construction,  while  a  simple  microscope,  if  bin- 
ocular, and  provided  with  all  desirable  adjustments,  might  be 
a  very  complicated  affair.  The  difference  between  simple  and 
compound  microscopes  is  this:  in  the  simple  microscope  we 
look  at  the  object  directly,  while  in  the  compound  microscope 
we  look  at  a  magnified  image  of  the  object.  In  the  simple 
microscope,  objects  are  always  seen  in  their  natural  position, 
while  in  the  compound  microscope  they  are  inverted,  and  right 
becomes  left,  and  left  becomes  right.  This  makes  it  very  diffi- 
cult for  beginners  to  work  upon  objects  under  the  compound 
microscope;  and  hence  simple  microscopes  are  almost  always 
used  for  dissecting  and  botanizing. 

It  is  true  that  by  adding  more  lenses,  and  making  the  instru- 
ment still  more  compound,  we  can  again  invert  the  image,  and 
thus  bring  it  back  to  its  original  and.  natural  position,  and 
almost  all  the  very  expensive  microscopes  are  furnished  with 
these  extra  lenses  arranged  in  a  piece  of  accessory  apparatus 
technically  known  as  an  erector.  The  distinguishing  feature  of 
the  compound  microscope  remains,  however,  the  same.  Certain 
forms  of  the  microscope,  in  which  concave  lenses  are  substi- 
tuted for  the  usual  convex  form,  also  give  erect  images,  but 
this  does  not  affect  the  general  truth  of  the  statement  just 
made. 

Simple  microscopes  frequently  consist  of  more  than  one 
lens.  Thus,  in  using  the  ordinary  pocket  magnifiers  with  two 
or  three  lenses,  it  is  usual  to  employ  all  the  lenses  at  once,  look- 
ing at  the  object  through  two  or  three  lenses  at  the  same  time 
when  a  high  power  is  required.  In  this  case,  however,  the  two 
or  three  lenses  are  placed  close  together  and  act  in  the  same 
way  as  a  single  lens,  with  surfaces  more  sharply  curved  than 
those  of  any  of  the  lenses  forming  the  combination.  Under 
such  circumstances  the  image  is  not  inverted,  but  if  we  now 
separate  the  lenses  sufficiently,  we  will  find  that  on  again  bring- 
ing the  object  into  focus,  the  image  is  inverted  and  greatly 
enlarged.  Moreover,  it  will  be  found  that  the  magnifying 
power  may  be  greatly  increased  by  increasing  the  distance  be- 
tween the  two  lenses,  and  it  will  also  be  found  that  as  the  dis- 


OF  THE  MICROSCOPE.  21 

tance  of  the  two  lenses  from  each  other  is  increased,  the  dis- 
tance at  which  the  combination  is  placed  from  the  object  must 
be  made  less  and  vice  versa.* 

The  early  forms  of  the  compound  microscope  consisted  of 
little  more  than  the  two  lenses  we  have  just  described,  but  the 
modern  instrument,  even  in  its  simplest  form,  is  a  vastly  more 
complicated  arrangement.  In  the  best  forms,  for  the  lens  next 
the  eye  there  is  substituted  an  eye-piece  consisting  of  two  lenses 
with  a  diaphragm  between  them,  while  the  objective,  or  lens  next 
the  object,  is  composed  of  from  four  to  ten  different  pieces  of 
glass,  forming  two  or  more  lenses,  which  are  so  arranged  that 
each  shall  correct  the  defects  of  the  others,  and  this  optical 
combination  is  mounted  on  a  stand  which  is  sometimes  a  mar- 
vel of  mechanical  ingenuity. 

*The  student  who  possesses  a  little  mechanical  genius  and  a  desire  to 
become  experimentally  acquainted  with  the  properties  of  lenses  and  the 
construction  of  the  microscope,  would  do  well  to  procure  a  couple  of  cheap 
lenses,  say  one  of  half  inch  focus,  and  one  of  about  two  inches  focus,  and  test 
by  actual  trial  the  statements  made  in  the  text.  Such  lenses  may  be  con- 
veniently arranged  in  a  tube  formed  of  writing  paper  and  gummed  on  the 
edges.  All  the  most  important  properties  and  defects  of  lenses  may  be  thus 
illustrated  and  studied.  By  means  of  a  little  extra  care,  two  such  lenses,  ar- 
ranged as  we  have  described,  in  tubes  blackened  on  the  inside,  and  mounted 
on  a  little  wooden  stand,  the  focus  being  adjusted  by  sliding  the  tube  hold- 
ing the  lenses  within  another  tube,  also  of  paper,  will  give  not  only  a  very 
fair  view  of  such  objects  as  the  wing  of  a  fly,  the  scales  on  a  butterfly's  wing, 
and  even  the  barbs  on  the  sting  of  a  bee,  but  it  will  show  the  globules  of 
blood  quite  distinctly,  and  we  have  even  given  a  very  interesting  exhibi- 
tion of  the  circulation  of  "the  blood  in  the  foot  of  a  frog  by  means  of  a 
temporary  arrangement  of  this  kind,  which  we  put  together  for  the  pur- 
pose of  explaining  to  a  little  girl  the  construction  of  the  microscope.  Wo 
would  not  recommend  any  one  to  use  such  a  microscope  for  purposes  of 
work  or  study,  because  the  fallacies  to  which  it  may  give  rise  are  too 
numerous  and  too  serious.  But  any  boy,  or  even  girl,  who  will  undertake 
the  construction  of  such  an  instrument,  cannot  fail  to  obtain  thereby  an 
amount  of  information  which  the  perusal  of  volumes  would  not  give.  As 
hints  towards  aiding  our  young  friends,  we  may  remark  that  our  tnbes 
were  made  of  the  best  stiff  paper,  rolled  up  tight  and  pasted  only  along 
the  outer  edge.  The  lenses  were  secured  in  their  places  by  being  attached 
to  the  bottoms  of  pill-boxes,  holes  being  punched  through  to  admit  the 
light.  Pill-boxes  with  holes  were  also  used  for  diaphragms  to  reduce  the 
effects  of  aberration.  A  piece  of  mirror  reflected  the  light,  and  the  sides 
etc.,  of  an  old  cigar  box  furnished  material  for  the  stand.  Fifty  cents 
covered  all  expenses. 


22  SELECTION  AND   USE 

Essential  Parts  of  the  Microscope.— When  a  good 
lens  is  held  steadily  at  a  certain  distance  from  an  object  which 
is  properly  illuminated,  this  distance  depending  upon  the  form 
and  material  of  the  lens,  we  are  enabled  to  see  the  object  clearly 
and  distinctly.  When,  however,  this  distance  is  either  in- 
creased or  diminished,  the  object  becomes  blurred  and  indis- 
tinct. The  point  at  which  vision  is  most  distinct  is  called  the 
focus*  of  the  lens,  and  when  we  are  able  to  see  it  clearly  the 
object  is  said  to  be  in  focus;  when  the  distance  is  either  in- 
creased or  diminished,  it  is  said  to  be  out  of  focus.  An  object 
is  said  to  be  within  the  focus  when  the  lens  is  too  near  it,  and 
beyond  the  focus  when  the  lens  is  too  far  away. 

The  performance  of  any  lens  depends  greatly  upon  the  ac- 
curacy with  which  it  is  adjusted  to  the  correct  focal  distance, 
and  the  steadiness  with  which  it  is  held  there.  For  all  ordi- 
nary purposes,  lenses  which  do  not  magnify  more  than  ten 
diameters  may  be  very  conveniently  held  in  the  hand  without 
any  special  means  of  support;  but  when  the  power  is  much 
greater  than  this,  or  where,  as  in  the  compound  instrument, 
the  microscope  is  bulky  and  heavy,  it  becomes  necessary  to  use 
some  mechanical  contrivance  which  will  hold  the  microscope 
steadily  in  its  position  in  relation  to  the  object,  otherwise  the 
view  becomes  indistinct.  Thus  a  good  lens,  magnifying  from 
thirty  to  forty  diameters,  will  very  readily  show  the  individual 
corpuscles  or  globules  in  the  blood  of  the  frog,  provided  it  is 
arranged  on  a  steady  support  and  accurately  adjusted  for  focus. 
But  if  the  lens  be  merely  held  in  the  hand,  without  any  firm 
support,  it  will  be  impossible  for  the  observer  to  see  the 
corpuscles. 

Another  important  point  is  the  illumination  of  the  object. 
Unless  the  object  be  properly  illuminated  it  cannot  be  dis- 

*  It  is  scarcely  necessary  to  inform  the  reader  that  the  focus  described 
in  the  text  is  not  precisely  the  focus  of  the  lens  itself,  but  the  focus  of  a 
compound  lens  of  -which  the  eye  forms  one  element.  Hence  the  focal 
distance  varies  with  different  eyes,  and  so  does  the  apparent  size  of  ob- 
jects. To  short-sighted  people  objects  appear  of  larger  size  than  they  do 
to  persons  of  ordinary  eye-sight.  In  working  with  the  compound  micro- 
scope we  frequently  find  that  different  people  require  a  different  focal 
adjustment. 


OF   THE   MICROSCOPE.  23 

tinctly  seen,  and  consequently  all  microscopes,  except  the 
simplest  forms  of  hand  magnifiers,  are  provided  with  means 
for  throwing  the  light  upon  the  object,  and  for  regulating  the 
amount  which  shall  either  fall  upon  it  or  pass  through  it. 

Hence  the  importance  of  providing  efficient  means  for  ad- 
justing the  focus  and  holding  and  illuminating  the  object,  and 
the  purpose  which  the  stand  is  intended  to  fulfil,  is  to  furnish 
these  means  in  a  compact  and  convenient  form.  Every  micro- 
scope, therefore,  of  a  grade  above  a  hand  magnifier,  whether 
it  be  simple  or  compound,  must  possess  : 

1.  Suitable  means  for  supporting  the  object,  and  placing  and 
maintaining  it  in  proper  position. 

2.  Means  for  illuminating  the  object,  either  by  throwing  the 
light  upon   it   when   it  is   opaque,  or  transmitting  the  light 
through  it  when  it  is  transparent. 

3.  Means  for  transmitting  to  the  eye  an  enlarged  image  of  the 
object. 

The  different  parts  which  are  employed  for  securing  these 
several  ends,  have  been  constructed  of  an  almost  endless  va- 
riety of  forms,  according  to  the  fancies  of  the  different  makers 
and  the  requirements  of  different  rnicroscopists.  As  it  is  im- 
portant that  the  student  should  familiarize  himself  with  the 
names  of  these  different  parts,  we  give  a  definition  of  them, 
and  in  prder  that  there  may  be  no  opportunity  for  mistake  or 
misapprehension,  we  have  also  engraved  an  outline  of  one  of 
the  ordinary  forms  of  the  microscope,  and  on  this  we  have 
marked  the  names  of  the  different  parts.  m 


NAMES  OF  THE  DIFFERENT  PARTS. 

The  following  are  the  names  of  the  essential  parts  of  a  com- 
pound microscope  of  ordinary  construction.  The  names  of  the 
different  parts  of  the  simple  microscope  are  the  same  as  those 
of  the  compound  microscope,  but  the  latter  has  several  parts 
which  do  not  exist  in  the  former. 

The  Stand  is  the  name  properly  applied  to  the  entire  frame 
used  for  supporting  and  illuminating  the  object,  and  for  carry- 
ing and  adjusting  the  optical  part,  the  latter  consisting  of  the 


SELECTION   AND   USE 
..Cap. 


Eye-piece 


Fig.   1.— DIAGRAM  SHOWING  THE   DIFFERENT  PARTS   OF  THE  MICRO- 
SCOPE,   AND  THEIR  NAMES. 


OF  THE  MICBOSCOPE.  25 

eve-piece  and  the  objective.  Stands  are  frequently  sold  separ- 
ately, or  furnished  with  eye-pieces  only — the  purchaser  making 
such  a  selection  of  objectives  and  other  accessories,  as  may  best 
suit  his  special  needs. 

The  Base  or  Foot  is  that  part  which'  supports  the  rest  of  the 
stand. 

The  Body  is  the  tube  to  which  the  eye-piece  and  objectives 
are  attached. 

A  Draw-tube  is  a  secondary  body  which  receives  the  eye-piece, 
and  slides  within  the  manTbody  like  the  draw  of  a  telescope. 
It  enables  us  to  increase  the  distance  between  the  eye-piece  and 
the  objective,  and  thus  to  change  the  magnifying  power,  as 
explained  in  a  previous  paragraph. 

A  Collar  is  a  short  tube  through  which  either  the  body,  the 
draw-tube,  or  the  eye-piece  slides. 

The  Arm  is  that  part  which  carries  the  body. 

The  Stage  is  the  plate  upon  which  the  object  is  placed  for 
examination. 

Clips  are  springs  attached  to  the  stage  for  tne  purpose  of 
holding  in  place  the  glass  slide  or  plate  carrying  the  object. 

A  Sub-stage  is  furnished  with  some  instruments.  Ijb  is  used 
for  holding  and  centering  various  means  of  illumination.  No 
sub-stage  is  shown  in  the  diagram,  Fig.  1.  The  sub-stage,  with 
its  centering  arrangements,  is  shown  very  olearly  in  the  Ross 
model,  Plate  I. 

Sub-stage  Ring. — Instead  of  being  provided  with  a  sub-stage, 
many  cheap  microscopes  are  furnished  with  what  is  known  as 
a  sub-stage  ring  o?'  tube.  This  serves  to  receive  the  polarizer, 
paraboloid,  etc. 

The  Object- Glass  or  Objective  is  the  lens  or  lenses  which  are 
placed  next  the  object.  The  term  Object-glass  is  sometimes 
applied  to  the  glass  plate  or  slide  upon  which  the  object  is 
placed,  but  this  use  of  the  word  is  entirely  wrong,  and  t^nds  to 
produce  confusion. 

The  Society  Screw  is  a  screw  of  a  certain  standard  size  for  con- 
necting the  objective  to  the  body.  In  microscopes  furnished 
with  a  screw  of  the  Society's  standard,  the  objectives  of  any 
American  or  English  maker  may  be  used.  The  makers  on  the 
continent  of  Europe  have  now  very  generally  adopted  the  So- 
ciety Screw.  In  this  country  the  Society  Screw  has  been  uni- 
versally adopted,  but  as  it  has  been  found  too  small  for  low- 
power  objectives  of  wide  angle,  another  screw,  named  the 


26  SELECTION    AND    USE 

Broad  Gauge,  is   sometimes   used   in   addition   to  the  Society 
Screw. 

In  all  microscopes,  means  are  provided  for  moving  the  objec- 
tive to  and  from  the  object,  so  as  to  bring  the  latter  into  focus, 
as  it  is  called.  According  as  the  device  used  for  this  purpose 
acts  coarsely  but  rapidly,  or  slowly  but  delicately,  it  is  called 
a  coarse  or  a  fine  adjustment.  The  best  microscopes  are  provided 
with  both  kinds,  so  that  the  object  is  first  brought  approxi- 
mately, but  rapidly,  into  focus  by  one,  and  then  adjusted  more 
slowly,  but  with  great  accuracy,  by  the  other. 

The  Coarse  Adjustment  has  several  forms.  In  Fig.  1  it  consists 
of  a  rack  and  pinion.  In  some  cases  it  is  a  chain  movement; 
very  often  it  is  effected  by  merely  sliding  the  body  up  and 
down  through  a  tubular  collar  by  hand,  as  in  Fig.  12. 

The  Fine  Adjustment  usually  consists  of  a  fine  screw,  some- 
times called,  improperly,  a  micrometer  screw,  which  moves  either 
the  entire  body  or  the  lower  part  of  it,  called  the  nose-piece. 
In  some  cheap  stands,  the  fine  adjustment  is  effected  by  moving 
the  stage  towards  the  objective. 

The  Nose-piece  is  a  short  tube,  which  fits  into  the  lower  end 
of  the  body,  and  carries  the  Society  screw  at  its  lower  end. 
Sometimes  it  is  made  to  slide  out  and  in,  and  thus  forms  part 
of  the  fine  adjustment.  In  the  instrument  figured  in  the  dia- 
gram, it  is  immovable.  The  term  nose-piece  is  also  applied  to 
certain  accessories  which  enable  us  to  attach  different  pieces  of 
apparatus  to  the  microscope,  as,  for  example,  two  or  more  ob- 
jectives at  one  time,  the  analyzer  of  the  polariscope,  etc. 

A  Diaphragm  is  usually  a  thin  plate  of  metal  pierced  with  a 
hole,  the  size  of  which  regulates  the  diameter  of  the  pencil  of 
rays  that  pass  through.  There  is  a  diaphragm  in  the  eye-piece 
which  contracts  the  field  of  view,  and  cuts  off  those  rays  which 
tend  to  confuse  the  images,  and  all  good  microscopes  have  a  dia- 
phragm attached  to  the  stage,  whereby  the  amount  of  light 
passing  through  the  object  to  the  objective  may  be  regulated. 
Instead  of  a  thin  plate  of  metal,  an  arrangement  known  as  the 
"Iris  diaphragm  "  is  used  in  the  microscope  shown  in  the 
diagram. 

The  Eye-piece  or  Ocular  is  the  short  brass  tube,  with  its  lenses, 
which  is  next  the  eye.  The  eye-piece  contains  an  Eye-Glass, 
which  is  that  next  the  eye  ;  a  Field-Glass,  placed  next  the 
objective,  and  a  Diaphragm,  consisting  of  a  brass  plate  with  a 
hole  through  it,  and  so  arranged  as  to  cut  off  the  outer  rays  of 
light.  The  tube  in  which  these  lenses  are  secured  is  in  almost 
all  cases  removable,  and  the  best  microscopes  are  furnished 


OF  THE  MICROSCOPE.  27 

•with  several  eye-pieces  of  different  powers,  which  may  be 
changed  at  pleasure.  We  may  here  remark  that  where  a  mi- 
croscope is  furnished  with  several  eye-pieces,  the  shortest  eye- 
piece gives  the  greatest  magnifying  power. 

The  Cap  is  a  cover  which  fits  over  the  top  of  the  eye-piece. 
It  is  pierced  with  a  hole,  to  allow  the.  rays  which  pass  through 
the  instrument  to  reach  the  eye.  This  cap  is  sometimes  covered 
with  a  second  cap,  which  has  no  hole,  but  which  is  intended 
to  exclude  dust  from  the  eye-lens,  and  which  is  removed  when 
the  instrument  is  in  use. 

The  Mirror  reflects  the  light,  and  causes  it  either  to  fall 
upon  the  object  or  to  pass  through  it,  so  as  to  render  it 
visible. 

The  Mirror-bar  is  a  bar  attached  to  the  stand  and  carrying 
the  mirror.  It  is  usually  made  to  turn  on  a  pivot,  and  in  the 
microscope  shown  in  Fig.  1,  it  consists  of  two  parts  sliding 
upon  each  other,  so  that  it  may  be  shortened  or  lengthened,  as 
circumstances  require. 

Accessories  are  those  parts  of  the  microscope  which  are  used 
only  for  special  purposes,  such  as  the  paraboloid,  the  camera 
lucida,  stage  forceps,  etc. 

The  Object  is  that  which  is  subjected  to  examination.  It  is 
usually  mounted  upon 

A  Slide,  or  plate  of  glass,  which  is  laid  upon  the  stage. 

All  these  parts  will  be  fully  described  in  a  subsequent  part 
of  this  volume,  and  their  utility  and  importance  explained.  It 
is  greatly  to  be  desired  that  the  young  microscopist  should 
familiarize  himself  with  the  terms  employed,  so  that  he  may 
always  use  them  accurately.  Anything  that  he  may  say  or 
write  will  then  be  clearly  understood  by  all  who  have  given 
careful  attention  to  the  subject. 


2<  SELECTION  AND  USE 


SIMPLE  MICROSCOPES. 

The  simple  microscope  is  an  indispensable  assistant  to  those 
who  use  the  compound  instrument,  as  well  as  to  those  who  rely 
upon  it  alone  for  the  examination  of  flowers,  seeds,  minerals, 
textile  fabrics,  etc.  We  shall  therefore  devote  some  space  to 
a  consideration  of  its  various  forms,  though  even  then  we  shall 
be  unable  to  do  more  than  describe  certain  typical  models 
which,  however,  afford  variety  enough  for  all  practical  pur- 
poses. 

Hand  Magnifiers.— These  are  so  generally  useful  and 
applicable  that  they  are  used  by  all  who  wish  a  cheap  and  yet 
efficient  aid  to  natural  vision.  They  are  found  in  market  in  a 
great  variety  of  forms,  styles  of  mounting,  and  price,  and  are 
too  well  known  to  need  minute  description.  Large  lenses, 
magnifying  two  or  three  times,  are  mounted  singly,  and  used 
chiefly  for  the  examination  of  pictures,  and  as  reading  glasses  ; 
the  smaller  sizes  of  the  same  style  serve  for  the  examination  of 
fine  engravings.  Very  small  lenses  of  considerable  power,  and 
simply  mounted  in  a  frame,  are  also  sold  by  most  opticians. 
They  are  known  as  "watch-charms,"  and  magnify  about  fifteen 
diameters.  We  have  also  seen  a  very  powerful  magnifier 
mounted  in  a  little  ring  attached  to  a  pair  of  eye-glasses. 

For  the  purpose  of  the  student  and  naturalist,  a  very  ex- 
cellent form  is  that  shown  in  Fig.  2.  It  consists  of  a  lens  of 
1  suitable  magnifying  power  set  in  a  frame,  which  folds  into  a 
case,  just  as  the  blade  of  a  pocket  knife  folds  into  its  handle, 
thus  allowing  the  instrument  to  be  carried  in  the  pocket  with- 
out liability  to  injury.  Similar  magnifiers  are  made  with  two, 
three,  and  even  four  separate  lenses,  as  shown  in  Fig.  3.  The 
lenses  are  usually  of  different  powers — a  2  inch,  1  inch,  and 
£  inch,  forming  a  very  "useful  combination.  Each  lens  may  be 
used  separately,  or  two  may  be  used  together,  or  all  three  may 


0?  THE  MICROSCOPE.  29 

be  used  at  once,  a  considerable  range  of  magnifying  power 
being  thus  secured. 


Fig.  2. — HAND  MAGNIFIER — SIMPLE  LENS. 

The  magnifiers  shown  in  Figures  2  and  3  are  furnished 
with  what  are  called  diaphragms — that  is.  to  say,  each  one  has  a 
thin  plate  of  some  opaque  material,  having  a  hole  of  suitable  size 
through  its  centre.  This  plate  is  placed  over  the  lens  when 


Fig.   3, — HAND   MAGNIFIER — THREE   LENSES. 

but  one  lens  is  used,  and  between  the  lenses  when  two  or  more 
are  employed.  It  serves  to  cut  off  the  marginal  rays  which  do 
not  give  a  clear  image,  and  in  this  way  it  greatly  improves  the 
definition  of  the  object. 

Two  or  more  lenses,  properly  adapted  to  each  other,  and  used 
together,  give  results  greatly  superior  to  anything  that  can  be 


30  SELECTION  AND  t?SE 

obtained  from  a  single  lens,  at  least  so  far  as  clearness  and 
accuracy  of  definition  is  concerned.  But  when  used  as  a  work- 
ing or  dissecting  microscope,  they  are  open  to  the  objection 
that  the  distance  at  which  they  must  be  placed  from  the  object 
is  very  small,  and  hence  it  is  frequently  inconvenient  to  use 
them  for  working  upon  objects.  Thus,  if  we  have  a  plano- 
convex lens  of  a  quarter  of  an  inch  focus,  and  one  of  three 
quarters  of  an  inch  focus,  and  place  them  at  a  distance  of  a  six- 
teenth of  an  inch  from  each  other,  we  will  have  a  very  good 
magnifier  which  will  enlarge  objects  about  fifty  times,  but  we 
must  place  it  at  but  a  very  short  distance  from  the  object.  If 
we  separate  the  lenses  a  little,  the  definition  will  be  improved, 
but  the  working  distance,  as  it  is  called,  will  be  diminished. 
Those  who  have  studied  optics  are  quite  familiar  with  these 
facts,  but  the  ordinary  reader  does  not  always  think  of  them, 
and  yet  they  are  very  important  when  we  come  to  choose  a  mi- 
croscope for  working  or  dissecting  purposes. 

Where  two  or  more  simple  lenses  are  used  together  (without 
being  combined  so  as  to  form  a  compound  microscope)  the 
power  of  the  combination  is  always  equal  to  the  sum  of  the 
powers  of  the  separate  lenses.  Thus  if  we  have  a  lens  of  half 
an  inch  focus  and  one  of  one  inch  focus,  one  magnifying  ten 
and  the  other  twenty  diameters,  the  resulting  power  is  thirty 
and  not  two  hundred  times.  In  the  compound  microscope,  on 
the  other  hand,  the  combination  of  an  objective  magnifying 
twenty  diameters  with  an  eye-piece  magnifying  ten  diameters, 
gives  a  magnifying  power  of  two  hundred  diameters. 

Watch-Makers'  Eye-Glasses.— These  are  well  known, 
and  may  be  obtained  of  almost  any  power  within  the  useful 
range  of  a  singe  lens.  They  are  called  "eye-glasses''  because 
when  in  use  they  are  held  by  the  muscles  around  the  orbif  oi 
the  eye,  and  consequently  require  no  extraneous  support. 
Fig.  4  shows  the  bell-mouthed  form  of  the  frame,  which  enables 
us  to  secure* this  result.  The  eye-glass  ordinarily  used  by 
watch-makers. magnifies  about  eight  times,  but  glasses  magnify- 
ing twenty  diameters  are  not  uncommon.  Glasses  of  the  latter 
power  are  usually  doublets,  that  is,  they  consist  of  two  lenses, 
arranged  together,  one  being  of  much  longer  focus  than  the 


OP  THE  MICROSCOPE.  31 

other.  If  well  made  they  give  excellent  definition  and  a  large 
field,  and,  when  mounted  on  a  stand,  are  very  serviceable  as 
dissecting  microscopes,  especially  in  working  upon  coarse  objects, 
and  picking  out  shells,  the  larger  foraminifera,  etc.  Their 
form  enables  us  to  support  them  by  means  of  a  small  wire 
ring,  arranged  as  in  a  retort  stand,  and  the  large  bell-mouth  of 
the  frame  prevents  any  light  from  entering  the  eye.  except  that 
which  has  passed  through  the  lens.  They  are  very  cheap,  and 
any  intelligent  boy  can  make  a  tolerable  stand  for  one.  The 
same  stand  will  answer  for  several  glasses  of  different  powers. 


Fig.  4.— WATCH-MAKERS'  GLASS.  Fig.  5. 

The  eye-glass  shown  in  Figure  4  is  a  doublet,  the  front  lens 
of  which  is  set  in  a  separate  piece,  shown  in  Figure  5.  When 
the  latter  is  removed,  the  remaining  part  forms  a  very  service- 
able low-power  glass  ;  when  both  are  used  together,  the  com- 
bination forms  an  excellent  high-power  magnifier.  This  form 
is  one  made  by  the  Bausch  &  Lomb  Optical  Co. 

Engravers'  Glasses.— These  are  mounted  in  frames, 
similar  to  that  of  the  watch-makers'  eye-glass,  but  as  they  are 
larger,  and  are  therefore  not  so  readily  held  in  the  eye,  after  the 
fashion  of  the  latter,  they  are  always  used  with  a  stand  of  some 
kind.  Those  of  the  best  quality  are,  in  general,  doublets,  con- 
sisting of  two  plano-convex  lenses,  and  as  they  give  a  large 
field  of  view,  with  very  good  definition,  they  are  altogether  the 


SELECTION  AND  USE 


best  microscopes  for  examining  bank  bills,  fine  engravings,  and 
similar  objects.     The  general  form  is  shown  in  Fig.  6. 


Fig.  6. — ENGRAVERS'  GLASS. 

Li  lie  11  ProTers.— These  are  a  very  old  form  of  the  simple 
microscope,  and  being  in  very  general  use,  they  are  manufac- 
tured in  large  quantities,  while  as  it  is  necessary  that  they 
should  be  of  good  quality  they  afford  an  opportunity  of  getting 
a  good  magnifier  at  a  moderate  price.  The  general  form  is 
shown  in  Fig.  7,  in  which  the  instru- 
ment appears  as  in  use.  The  upper 
plate  carries  the  lens,  and  the  length  of 
the  upright  is  such  that  when  the  base 
is  placed  upon  any  flat  surface,  that 
surface  will  be  in  focus.  The  base  is 
pierced  with  a  hole  one  quarter  of  an 
inch  square,  and  when  placed  on  a 
piece  of  cloth  the  lens  enables  us  to 
count  the  number  of  threads  which  oc- 
cupy that  space.  This  indicates  the 
fineness  of  the  fabric. 


f.  7. — LINEN  PROVER. 


The  Stanhope  Lens  consists  of  a  cylinder  or  rod  of 
glass,  one  end  of  which  is  rounded  so  as  to  form  a  lens,  while 
the  other  end  is  either  flat  or  slightly  curved.  The  distance 


•    OF  THE  MICROSCOPE,  33 

between  the  lens  and  the  flat  surface  is  exactly  equal  to  the 
focal  distance  of  the  lens.  Transparent  objects,  such  as  the 
scales  of  insects,  animalculse  in  water,  etc.,  are  simply  placed 
on  the  flat  surface  of  the  glass  cylinder,  and  when  looked  at 
through  it,  they  appear  greatly  magnified.  It  is  easily  used, 
but  can  not  well  be  employed  as  a  working  microscope.  It  is 
this  kind  of  lens  that  is  used  in  the  construction  of  those  watch 
charms  in  which  a  large  picture  is  seen  on  looking  through  a 
very  small  hole.  The  picture  is  a  photograph  attached  to  the 
flat  end  of  a  small  glass  rod,  the  other  end  of  the  rod  being 
formed  into  a  lens  of  exactly  the  right  focal  length  required  to 
show  the  picture  clearly  and  considerably  magnified.  Lenses 
and  photographs  of  this  kind  are  usually  mounted  as  miniature 
opera-glasses. 

The  Stanhope  lens  seems  to  be  a  favorite  in  France,  where  it 
is  manufactured  very  extensively,  and  sold  under  the  name  of 
the  Stanhoscope.  One  advantage  claimed  for  it  is  that  no  adjust- 
ment for  focus  is  required,  the  flat  surface  of  the  lens  being 


c 

Fig.   8. — STANHOPE   COLLECTING  LENS. 

exactly  in  the  focus  of  the  curved  surface.  This  feature,  while 
presenting  some  slight  advantages,  has  also  the  great  disad- 
vantage that  objects  covered  with  thin  glass  cannot  be  ex- 
amined by  such  lenses,  and  neither  can  objects  having  a 
sensible  thickness.  The  feature  which  we  have  just  mentioned 
is  imitated  in  some  cases  by  placing  a  piece  of  thin  glass  in 
front  of  a  small  lens,  and  at  such  a  distance  that  the  outer  sur- 
face of  the  thin  glass  will  be  exactly  in  focus,  as  is  hereafter 
described  when  peaking  of  the  Craig  microscope. 

There  is  one  purpose,  however,  to  which  this  form  of  lens  is 
applied  with  good  effect,  and  that  is  to  the  construction  of  a 
"  collecting  lens  "  as  it  is  called — that  is,  a  lens  for  examining 
drops  of  water  when  searching  for  diatoms,  algae,  animalculae, 
etc.  Fig.  8  shows  a  lens  of  this  kind  made  by  Jas.  "W.  Queen 
&  Co.,  of  Philadelphia.  The  lens  is  set  in  a  brass  frame,  pro- 


34  SELECTION  AND   USE      ' 

vided  with  a  handle,  and  qiter  a  drop  of  water  has  been  placed 
on  the  flat  surface  of  the  lens,  the  cap,  c,  is  screwed  on,  and  the 
object  examined  by  simply  holding  the  instrument  up  to  the 
light. 

A  very  excellent  collector's  microscope  will  also  be  found  de- 
scribed amongst  the  compound  instruments. 

The  Coddington  Lens.— This  lens  was  devised  by  Sir 
David  Brewster,  but  having  been  made  by  a  London  optician 
for  Mr.  Coddington,  it  was  called  by  his  name,  which  has  stuck 
to  it  ever  since.  It  consists  of  a  cylinder  of  glass,  the  two  ends 
of  which  have  been  ground  so  as  to  form  portions  of  the  surface 
of  the  same  sphere.  A  deep  groove  is  cut  around  the  cylinder, 
midway  between  the  ends,  and  a  diaphragm  is  thus  formed 
between  the  two  lenses.  In  Figure  9  is  shown  a  very  neat 
and  convenient  method  of  mounting  the  Coddington. 

This  form  of  lens  gives  very  sharp  definition,  so  that  when- 
ever a  power  greater  than  twenty  diameters  is  required  for 
examining  objects,  a  Coddington,  if  well  made,  will  be  found  to 

be  the  best  lens  in  use,  always,  of 
course,  excepting  the  carefully 
corected  doublets  and  triplets 
hereafter  described.  The  price 
of  the  latter,  however,  is  in 
general  four  to  eight  times  that 
of  a  good  Coddington.  But  it 
has  this  defect,  that  the  working 
Fig.  9.— CODDINGTON  LENS.  focus  is  very  short,  and  therefore 
for  a  dissecting  microscope  a 

doublet  is  to  be  preferred.  In  using  a  Coddington  lens,  great 
care  must  be  taken  to  secure  good  illumination  of  the  object, 
and  the  shortness  of  the  focus  makes  this  difficult  to  those  who 
have  had  no  experience.  4* 

Those  who  desire  to  acquaint  theinselvas  with  the  structure 
and  peculiarities  of  the  most  important  simple  microscopes, 
will  find  this  subject  very  fully  and  very  clearly  treated  in  the 
article  contributed  by  Andrew  Ross  to  the  "  Penny  Cyclopaedia," 
published  by  the  Society  for  the  Diffusion  of  Useful  Knowl- 
edge. This  article  has  been  republished  in  book  form. 


OF  THE  MICROSCOPE.  35 

Achromatic  Doublets  and  Triplets.— Magnifiers, 
composed  of  two  or  more  lenses,  are  to  be  had  of  two  very 
distinct  kinds.  The  lenses  may  either  be  simply  united  in  one 
frame,  without  any  special  adaptation  to  each  other,  or  the  in- 
strument may  consist  of  two  or  more  achromatic  lenses  com- 
bined together  in  a  fixed  and  accurately  determined  relation. 
Examples  of  the  former  are  found  in  the  ordinary  two  and  three 
lens  magnifiers  we  have  just  described  ;  the  latter  are  not  so 
common,  since  they  are  somewhat  expensive  when  well  made. 
They  are  known  as  achromatic  doublets  and  triplets,  and  one 
maker  in  this  country,  Mr.  Tolles,  of  Boston,  has  long  been 
noted  for  the  excellence  of  the  simple  achromatic  microscopes 
of  this  class  made  by  him. 

The  advantages  possessed  by  lenses  of  this  kind  are  the  larger 
field  of  view  which  they  give,  thus  enabling  us  to  obtain  a  clear 
view  of  objects  of  considerable  size  ;  and  the  wonderfully  in- 
creased sharpness  of  definition  which  they  afford,  owing  to  their 
wide  angular  aperture.  They  are  usually  mounted  in  the  same 
style  as  the  Coddington  (Fig.  9),  but  are  readily  distinguished 
from  the  latter  by  the  absence  of  any  diaphragm.  The  lenses 
of  which  these  magnifiers  are  composed  are  so  constructed  and 
combined  that  the  field  is  perfectly  flat  and  well  defined  in  all 
its  parts,  so  that  a  diaphragm  is  not  necessary.  Mere  common 
lenses,  put  together  so  as  to  resemble  these  achromatic  doublets, 
and  without  a  diaphragm,  would  give  such  a  misty  view  of  ob- 
jects that  they  would  be  at  once  condemned. 

With  the  exception  of  the  low  and  medium-power  achromatic 
objectives  used  with  the  compound  microscope,  these  doublets 
and  triplets,  when  well  made,  are  altogether  the  most  satisfac- 
tory simple  microscopes  in  use,  and  several  firms  now  make  a 
specialty  of  their  manufacture.  Amongst  others  we  may  name 
the  Bausch  &  Lornb  Optical  Company,  Messrs.  K.  &  J.  Beck, 
Browning,  whose  magnifiers  of  this  kind  are  known  as  Platy- 
scopic  lenses,  and  Steinheil,  of  Munich. 

To  some  it  may  appear  that  we  have  devoted  more  space  to 
the  simple  microscope  than  its  importance  demands.  Our 
excuse  is  that  simple  microscopes,  of  the  different  kinds  we 
have  just  described,  are  not  in  such  general  use  as  they  ought 
to  be.  This,  however,  seems  to  be  the  case  even  in  France,  the 


36  SELECTION  AND  USE 

country  of  Baspail,  wlio  was  the  great  advocate  of  the  simple 
microscope,  for  Chevalier,  in  his  work,  says  :  "It  is  a  matter  of 
regret,  from  a  scientific  point  of  view,  that  the  simple  micro- 
scope is  not  more  extensively  used  in  France  than  it  is,  because 
in  minute  dissections  it  is  capable  of  rendering  immense 
service." 

Twenty-five  cent  Microscopes.— Before  leaving  this 
subject  it  may  be  well  to  say  a  few  words  about  those  very  cheap 
microscopes  which  have  been  so  extensively  advertised.  We 
frequently  see  in  the  papers  an  advertisement  in  which  some 
person  offers  to  send  for  twenty-five  cents  a  microscope  which 
will  show  animalcules  in  water,  globules  of  blood  etc. ,  etc.,  and 
the  question  naturally  arises,  Are  these  microscopes  good  for 
anything,  or  is  the  advertisement  a  swindle — the  advertiser 
taking  the  money  and  sending  nothing  in  return? 

As  a  general  rule,  those  who  send  to  such  advertisers,  receive 
in  return,  a  plate  of  brass  or  lead,  with  a  glass  bead  fastened 
in  a  hole  in  the  centre.  The  glass  bead  is  formed  by  fusion  and 
is  frequently  ground  flat  and  polished  on  the  side  by  which  it 
•was  attached  tj  the  thread  or  rod  of  glass  from  which  it  was 
made,  forming  in  such  cases  a  hemispherical  lens.  Such 
lenses  are  very  easily  made  by  any  one.  Take  a  strip  of  flint 
glass,  such  as  a  piece  of  flint  glass  tubing,  or  a  piece  of  glass 
rod,  draw  it  out  to  a  thread  in  the  flame  of  a  spirit  lamp, 
fuse  the  end  and  allow  it  to  gather  into  a  drop.  Give  plenty  of 
time  and  a  good  strong  heat,  so  that  the  surface  of  the  little 
globe  may  become  well-fused  and  truly  round.  The  best  re- 
sults are  always  obtained  by  holding  the  thread  perpendicularly , 
as  when  held  horizontally  the  globule  is  apt  to  become  dis- 
torted. Make  one  or  two  dozen  of  these,  and  in  separating 
them  from  the  glass  rod  leave  about  an  eighth  of  an  inch  of 
the  latter  attached  to  each  globule,  to  serve  as  a  handle,  in  the 
next  step  of  the  process,  which  consists  in  inserting  them  to 
about  half  their  depth  in  a  plate  of  cement,  consisting  of 
shellac  thickened  with  very  dry  and  finely  powdered  pumice- 
stone.  To  form  such  a  wax  plate,  melt  some  shellac  in  a  ladle 
or  large  iron  spoon,  mix  it  carefully  with  as  much  powdered 
pumice-stone  as  can  be  conveniently  stirred  in,  remove  it  from 


OF    THE   MICKOSCOPE.  37 

the  fire,  stir  well  until  it  begins  to  stiffen,  and  then  pour  it  out 
on  a  flat  metal  plate — the  surface  of  a  smoothing  iron  answer- 
ing very  well.  The  plate  of  cement  should  be  from  one-half  to 
three-quarters  of  an  inch  thick,  and  the  little  globules  are  easily 
fastened  into  it  by  seizing  them  by  the  small  handles  left  on 
them,  holding  them  by  a  pair  of  forceps  in  a  lamp  flame  until 
they  are  hot  enough  to  melt  the  cement,  and  then  pressing 
them  m  to  about  half  their  depth  or  a  little  more.  When  quite 
cold  they  will  be  very  securely  held.  The  little  handles,  or  tails, 
are  now  nipped  off  with  a  pair  of  cutting  pliers,  and  the  glo- 
bules ground  all  at  once  on  a  fine  grindstone,  or  still  better  on 
a  metal  plate  charged  with  emery.  When  they  have  been  re- 
duced nearly  to  the  surface  of  the  plate  of  cement,  they  should 
be  ground  with  emery  of  the  finest  kind,  and  as  soon  as  all 
coarse  scratches  have  been  removed  they  should  be  polished  on 
a  buff  leather  with  crocus  martis  or  putty  powder.  When  finely 
polished  they  may  be  removed  from  the  cement  by  means  of  a 
small  chisel,  and  any  cement  that  adheres  may  be  dissolved  off 
by  means  of  alcohol.  They  are  then  mounted  in  thin  plates  of 
lead,  brass,  or,  what  is  better  still,  vulcanite.  Out  of  two  dozen 
such  globules,  carefully  made  and  well  polished,  three  or  four 
may  be  obtained  that  will  give  satisfactory  definition,  and  it 
was  with  such  lenses  that  the  early  microscopists  made  many  of 
their  discoveries.  These  men,  however,  took  great  pains  in 
making  and  polishing  them,  and  rejected  hundreds  as  unfit  for 
use.  The  objections  to  the  microscopes  of  this  kind,  that  are 
ordinarily  sold,  are  that  they  are  badly  made,  and  that  good 
and  bad  are  sold  together  without  any  selection  being  exercised. 
But,  even  if  well  made,  they  are  very  difficult  to  use,  and  very 
unsatisfactory  in  their  results,  even  in  the  hands  of  persons  of 
great  skill.  The  polish  of  a  fused  surface  never  equals  that  of 
a  surface  finely  cut  aud  polished,  as  every  housekeeper  that  is 
familiar  with  common,  and  with  cut  glass,  very  well  knows. 
The  fused  surface  of  these  little  globes  is,  therefore,  always 
more  or  less,  covered  with  striae  or  very  minute  ridges  which 
interfere  with  their  defining  powers,  and  we  have  described 
thus  minutely  the  process  of  their  manufacture,  rather  for  the 
purpose  of  giving  our  readers  such  information  as  will  enable 
them  to  understand  how  they  can  be  sold  so  cheaply,  than  in 
the  hope  that  they  will  endeavor  to  make  them  for  themselvc  s. 


38  SELECTION    AND    USE 

Penny  Microscopes. — A  few  years  ago  a  man  in  London 
made  a  living  by  selling  through  the  streets  a  microscope  which 
would  show  the  eels  in  paste  and  vinegar,  and  of  which  the 
price  was  only  one  penny,  (equal  to  two  cents.)  These  micro- 
scopes were  thus  made :  In  the  bottom  of  a  pill-box  he  punched 
a  small  hole  and  then  blackened  the  inside  of  the  box.  In  this 
hole  was  placed  a  drop  of  Canada  balsam  or  damar  varnish, 
which  was  allowed  to  dry.  When  hard,  the  balsam  formed  a 
very  tolerable  lens. 

A  drop  of  water,  balsam,  or  varnish,  laid  on  the  under  side 
of  a  slip  of  clear  glass  will  often  enable  us  to  extemporize  a 
microscope  capable  of  doing  good  service  in  the  hands  of  a 
skillful  observer.  The  outline  of  the  drop  should  be  perfectly 
round,  and  the  glass  plate  should  be  held  as  level  as  possible.  We 
have  derived  great  assistance  from  such  a  lens,  when  better 
could  not  be  had. 

The  Craig  Microscope.— This  microscope  at  one  time 
attained  an  unprecedented  degree  of  popularity,  not  on  account 
of  its  merits,  but  because  of  the  extensive  puffing  and  adver- 
tising which  it  received.  It  consists  of  a  vertical  frame,  some- 
what like  that  of  the  cheap  French  microscopes,  liaviug  a 
mirror,  but  no  sliding  tube,  as  there  is  no  occasion  for  any.  The 
slide  which  holds  the  object  is  slipped  through  a  horizontal  slit 
cut  in  the  stand,  and  the  lens  with  its  frame  is  laid  on  it. 

The  lens  is  a  fused  bead  of  glass  set  in  a  little  frame,  to  the 
under  side  of  which  is  attached  a  thin  plate  of  glass,  whose 
lower  surface  is  exactly  in  the  focus  of  the  bead,  so  that  when 
a  drop  of  water  or  vinegar  is  placed  on  the  glass  plate,  or  such 
objects  as  insects'  scales,  wings,  etc.,  are  laid  on  it,  they  are  ex- 
actly in  focus.  Hence,  this  microscope  is  said  to  require  no 
adjustment  for  focus.  This  is  true  when  the  objects  to  be  ex- 
amined are  actually  in  contact  with  the  glass  plate,  but  when 
we  wish  to  examine  objects  that  are  covered  with  thin  glass  (as 
all  valuable  preparations  should  be)  or  objects  having  a  percep- 
tible thickness,  it  is  impossible  to  adjust  it  for  focus,  and  hence 
it  is  impossible  to  examine  such  objects  satisfactorily.  Besides 
this,  nine-tenths  of  the  microscopes  of  this  pattern  in  market, 
are  very  badly  made,  and  distort  objects  to  such  au  extent  that 


OF    THE   MICEOSCOPE.  39 

one  if  ho  has  been  accustomed  to  employ  a  good  microscope 
cannot  recognize  them.  It  has  unquestionably  done  a  great 
great  deal  to  impede  the  progress  of  microscopy  in  this  country, 
and  we  have  been  led  to  give  this  extended  description  of  it, 
chiefly  because  so  many  editors  and  clergymen  have  praised  it 
in  the  highest  terms.  It  has  even  been  patented,  although  the 
principle  upon  which  it  is  constructed  is  very  old;  but  then  we 
must  remember  that  under  our  present  administration  the 
patent  office  seems  to  be  conducted  rather  for  the  discourage- 
ment than  the  encouragement  of  progress  and  invention.  We 
daily  see  patents  issued  for  old  and  worthless  devices,  while  it 
is  well-known  that  the  author  of  a  really  meritorious  inven- 
tion will  have  the  hardest  work  to  obtain  protection. 

Of  the  Novelty,  Globe,  and  other  similar  microscopes,  it  is 
unnecessary  to  speak.  In  all  the  microscopes  of  this  kind  that 
we  have  seen,  the  optical  part  is  utterly  worthless.  The  lenses 
are  mere  fused  globules  of  glass,  and  they  distort  beyond  recog- 
nition the  image  of  any  object. 

Strange  to  say,  however,  even  this  fact  has  been  used  as  an 
argument  to  sell  them.  They  have  been  sold  chiefly  by  news- 
dealers and  stationers,  and  as  the  purchasers  did  not  know  how 
any  given  object  ought  to  appear,  the  fact  that  it  looked  so  very 
different  from  what  they  expected  was  considered  an  evidence 
of  the  power  of  the  microscope  ! ! 

In  regard  to  all  microscopes  in  which  fused  globules  are 
used,  it  must  be  remembered  that  the  lower  the  power  of  the 
lens  the  more  apt  it  is  to  be  imperfect.  No  lens  of  this  kind, 
magnilying  from  100  to  150  times,  (according  to  the  estimates 
of  those  who  deal  in  them,  which,  however,  is  in  fact  only  from 
ten  to  twelve  times^  as  measured  by  proper  methods),  can  be 
good  for  anything.  On  the  other  hand,  it  must  be  borne  in 
mind  that  when  we  attempt  to  examine  objects  under  high 
powers,  obtained  by  the  use  of  very  small  single  lenses,  we 
subject  our  eyes  to  an  almost  destructive  strain. 


40  SELECTION  AND   USE 


DISSECTING  MICROSCOPES. 

The  term  Dissecting  Microscope  is  applied  to  all  microscopes 
used  for  working  upon  objects  under  moderate  magnifying 
powers.  They  are  used  not  only  for  dissecting,  properly  so 
called,  but  for  the  study  of  botany,  mineralogy,  etc.,  as  well  as 
for  numerous  investigations  in  the  arts.  A  good  microscope  of 
this  kind  is  absolutely  indispensable  to  those  who  hope  to  do 
more  than  merely  look  at  objects  prepared  by  others. 

For  ordinary  purposes  of  examination,  the  different  magni- 
fiers previously  described  serve  very  well  when  merely  held  in 
the  hand,  but  their  performance  is  greatly  improved  when  they 
are  mounted  on  appropriate  stands,  which  not  only  enable  us 
to  adjust  the  focus  with  great  accuracy,  but  which  hold  the 
lens  steadily  in  relation  to  the  object,  and  thus  prevent  any 
necessity  for  that  constant  adjustment  of  the  eye  itself,  which 
always  occurs  when  a  lens  trembles,  and  which  is  so  fatiguing 
and  injurious  to  the  sight.  A  complete  dissecting  micro- 
scope should,  therefore,  be  furnished  with  stand,  mirror, 
etc. ,  and  if  the  student  can  afford  it,  there  should  be  some  good 
mechanical  means  of  adjusting  the  focus.  The  importance  of 
dissecting  microscopes  being  generally  recognized,  even  if  the 
instruments  themselves  have  not  come  into  general  use,  nearly 
every  manufacturer  of  microscopes  has  devised  a  stand  of  his 
own,  some  of  which  are  very  complete,  and  as  a  consequence 
very,  expensive.  We  shall,  therefore,  rest  content  with  de- 
scribing a  few  of  the  simpler  forms. 

The  microscopist  can  easily  construct  a  dissecting  microscopo 
for  himself,  by  means  of  a  cork  and  some  wire,  for  whenever  a 
piece  of  apparatus  is  to  be  supported  steadily,  while  at  the 
same  time  it  is  necessary  that  it  should  be  easily  moved  and 
adjusted,  nothing  serves  so  well  for  a  temporary  expedient  as  a 
fine  cork  sliding  on  a  smooth  wire.  Consequently  a  very 
serviceable  stand  for  a  simple  microscope  is  easily  extemporized 


OF   THE   MICROSCOPE.  41 

as  follows :  Procure  a  good  sound  -wine  cork,  and  bore  two 
holes  through  it,  the  holes  being  at  right  angles  to  each  other, 
and  to  the  axis  of  the  cork.  The  holes  should  be  of  the  right 
size  to  slide  easily,  but  firmly,  on  a  wire  rather  more  than  the 
sixteenth  of  an  inch  in  diameter.  One  piece  of  such  wire  is 
stuck  perpendicularly  in  a  wooden  foot,  and  serves  as  a  stand 
upon  which  the  cork  slides  up  and  down;  another  piece  of  wire, 
having  a  ring  at  one  end  for  holding  the  magnifier,  is  thrust 
through  the  second  hole  in  the  cork,  and  is  supported  by  it  in 
a  horizontal  position.  The  horizontal  wire  should  be  so  long 
that  there  will  be  no  danger  of  injury  to  the  eye  or  face  from 
the  upright  wire.  A  piece  of  white  paper  makes  the  best 
mirror,  as  it  gives  sufficient  illumination  with  low  powers,  and 
besides  this,  it  diffuses  the  light  very  equally  over  the  object. 
This,  of  course,  is  for  use  with  transparent  objects  ;  when 
opaque  objects  are  under  examination  a  condensing  lens  must 
be  employed. 

Tlie  Excelsior  Micro =cope.— The  accompanying  en- 
graving gives  a  very  clear  view  of  this  microscope,  which  is 
constructed  as  follows : 

To  one  end  of  the  lid  of  a  small  wooden  case  or  box,  is  at- 
tached one  of  the  ends  of  the  box;  and  when  the  lid  is  reversed 
and  turned  upside  down,  it  may  be  slid  into  the  groove  which 
usually  receives  it,  and  then  forms  a  stand  for  the  lenses  and 
glass  stage,  as  is  shown  in  engraving.  The  lenses  and  stage  are 
supported  by  a  steel  rod,  D,  the  lower  end  of  which  is  hinged 
to  the  lid,  so  that  it  may  be  turned  down  and  lie  in  a  groove 
provided  for  it.  When  raised  into  the  position  shown  in  the 
figure,  it  is  held  very  securely  in  place  by  means  of  the  button, 
E;  and  this  button  also  serves  to  retain  it  in  the  groove  when 
it  is  turned  down.  The  glass  stage,  G,  which  is  fitted  into  a 
frame  of  hard  rubber,  slides  easily  on  the  stem,  D,  so  as  to  be 
readily  adjustable  for  focus,  while  at  the  same  time  it  may  be 
firmly  fixed,  by  means  of  a  set-screw,  at  eny  desired  height, 
and  will  then  serve  as  a  stage  for  dissecting  purposes.  The 
frame  which  holds  the  lenses  fits  on  to  the  top  of  the  stem.  A 
mirror,  H,  is  fitted  into  the  case,  and  is  readily  adjustable  by 
means  of  the  button  shown  on  the  outside,  so  that  light  may 


42 


SELECTION    AND    USE 


be  reflected  up  through  the  stage  when  the  objects  to  be  ex- 
amined are  transparent;  and  when  they  are  to  be  viewed  by  re- 
flected light  there  is  a  dark  ground  of  hard  rubber  (not  shown 
in  the  engraving)  which  is  also  carried  by  the  stem,  D,  and 
may  be  turned  under  the  stage,  so  as  to  cut  off  all  transmitted 
light.  Dissecting  needles  (K  and  L),  with  neat  handles,  fit 
into  appropriate  grooves.  When  the  lenses  and  stage  are  re- 
moved from  the  stem  they  are  readily  packed  in  the  case;  the 


Fig.   10. — THE  EXCELSIOR  MICROSCOPE. 


stem  is  then  turned  down  and  held  in  its  groove  by  the  button, 
E;  the  lid  is  drawn  out  of  the  groove,  turned  over,  and  re- 
placed so  that  the  vertical  piece  (C)  closes  the  open  end  of  the 
box,  and  the  whole  thing  is  packed  into  a  compass  which  readily 
admits  of  its  being  carried  in  the  vest  po'ket. 

The  lenses  are  well  made,  and  being  provided  with  a  proper 
diaphragm,  great  clearness  of  definition  is  secured.     Two  styles 


OF    THE  MICROSCOPE.  43 

of  frame  are  sold,  one  containing  two,  and  the  other  three 
lenses,  the  latter  being  altogether  the  cheapest,  in  proportion 
to  the  power  furnished.  The  magnifying  powers  are  about  as 
follows:  With  the  lens  of  longest  focus,  five  diameters;  with 
the  lens  of  medium  focus,  eight  diameters;  with  the  lens  of 
shortest  focus,  ten  diameters.  When  the  lenses  of  shortest  and 
medium  foci  are  combined  the  magnifying  power  is  about 
eighteen  diameters;  all  three  lenses  together  give  a  power  of 
twenty-five  to  thirty  diameters. 

In  using  a  combination  of  two  or  more  lenses,  the  lens  of 
shortest  focus  should  always  be  placed  nearest  to  the  object. 

As  a  dissecting  microscope  for  botanical,  entomological,  and 
physiological  work,  this  instrument  is  very  efficient  and  con- 
venient. The  glass  plate  is  fitted  into  the  stage  so  as  to  form 
a  cell  capable  of  holding  water,  so  that  dissections  may  be 
carried  on  under  that  liquid,  or  aquatic  animals  may  be  kept 
alive  and  examined  at  leisure.  The  stage  may  also  be  turned, 
so  that  the  flat  side  will  be  up  when  so  desired,  in  which  posi- 
tion it  is  most  convenient  for  some  purposes,  such  as  dissec- 
tions and  the  teasing  out  of  tissues  by  means  of  needles.  The 
only  serious  defect  in  the  Excelsior  microscope  is  that  it  is  not 
sufficiently  steady  for  ordinary  work,  the  case  which  forms  the 
base  or  foot  being,  for  portability's  sake,  made  quite  small. 
This  difficulty  is,  however,  easily  remedied  by  screwing  the 
case  to  a  piece  of  pine  board  six  inches  long,  four  inches  wide, 
and  three-quarters  of  an  inch  thick.  A  single  small  screw, 
which  does  not  deface  the  instrument,  is  sufficient,  and  when 
the  microscope  is  to  be  carried  in  the  pocket  it  is  easily  de- 
tached from  its  temporary  stand.  Its  low  price,  $2.75,  is  a 
strong  recommendation. 

Raspail's  Microscope.— This  was  the  instrument  so 
largely  used  by  Raspail  in  his  investigations  into  the  structure 
of  plants,  and  having  been  fully  described  by  him,  it  has  been 
called  by  his  name,  though  Chevalier  tells  us  that  it  was  reallv 
invented  by  Cuff,  of  London,  r.nd  ought  to  be  called  the  Cuff 
microscope.  It  consists  of  a  pillar  which  screws  into  the  top 
of  the  box  in  which  I  he  whole  instrument  packs  when  the  micro- 
scope is  not  in  use.  This  pillar  carries  the  mirror,  and  also  a 


4A  SELECTION  AND  USE 

fixed  stage.  It  is  hollow,  and  in  it  slides  a  rod  which  may  be 
moved  up  and  down  by  means  of  a  rack  and  pinion,  so  as  to 
adjust  for  focus.  This  rod  carries  a  horizontal  arm,  into  one 
end  of  which  the  lenses  fit,  and  the  arm  itself  may  be  moved 
back  and  forth  across  the  top  of  the  vertical  rod,  so  as  to  bring 
the  lens  over  any  part  of  the  object.  The  great  objection 
to  this  microscope,  as  usually  constructed,  is  its  want  of 
steadiness,  and  this  is  a  vital  defect. 

The  Compact  Dissecting  Microscope.— Those  who 
desire  a  really  serviceable  and  substantial  microscope  for  dis- 
secting purposes,  will  find  that  several  manufacturers  have 
recently  endeavored  to  supply  the  demand  for  an  instrument 


Fig.   11. — THE  COMPACT  DISSECTING  AND  MOUNTING  MICROSCOPE. 

of  this  kind  at  a  moderate  price.  We  select  one  by  the  same 
firm  that  manufactures  the  Excelsior  Microscope — the  Bausch  & 
Lomb  Optical  Co.  It  is  called  the  "Compact  Dissecting  and 


OF   THE   MICROSCOPE.  45 

Mounting  Microscope,"  from  the  fact  that  it  may  be  folded 
together  so  as  to  occupy  very  little  space,  and  may  then  be 
packed  in  the  case  which  contains  the  compound  microscope, 
or  carried  in  its  own  little  box.  It  is  capable  of  receiving 
lenses  of  long  focus,  as  the  rod  which  carries  the  horizontal 
arm  slides  up  and  down  in  a  tube  upon  which  a  rack  is  cut. 
This  sliding  movement  serves  for  a  coarse  adjustment,  the  rack 
and  pinion  just  mentioned  serving  for  the  fine  adjustment. 
The  mirror  is  hung  with  all  necessary  movements,  and  the  arm 
which  carries  the  lenses  is  provided  with  the  Society  screw,  so 
that  ordinary  achromatic  objectives  may  be  used — thus  saving 
the  expense  of  extra  lenses. 

The  engraving  fails  to  show  an  important  feature  which  is 
provided  by  the  makers.  This  is  a  joint  in  the  arm,  which,  in 
connection  with  the  rotation  that  may  be  given  to  the  rod  to 
which  the  arm  is  attached,  enables  the  operator  to  place  the 
lens  over  any  part  of  the  preparation.  We  call  this  an  im- 
portant feature,  for  we  hold  it  to  be  essential  to  the  convenient 
use  of  the  dissecting  microscope  that  the  preparation  should 
remain  stationary  and  firm  during  the  processes  and  operations 
to  which  it  is  subjected.  This  is  just  the  reverse  of  what  is 
found  to  be  most  convenient  and  efficient  in  the  compound 
microscope.  There  the  objective  should  be  immoveable  (hori- 
zontally), and  the  object  should  be  adjusted  to  a  proper  posi- 
tion in  relation  to  it. 

Rests  for  the  arms,  when  using  the  dissecting  microscope,  are 
easily  improvised  out  of  books  or  blocks  of  wood.  \Ve  prefer, 
to  anything  else,  good-sized  blocks  of  wood,  having  the  form  of 
a  wedge  with  the  point  cut  off.  They  are  clumsy,  it  is  true, 
but  very  comfortable.  Small  rests  may  be  fitted  like  wings  to 
each  side  of  the  stage,  and  these  are  furnished  at  a  slight  extra 
cost  by  the  makers  of  the  microscope  just  described. 

Cheaper  forms  of  this  microscope  are  also  made.  In  these 
the  instrument  is  not  made  to  fold  up,  and  there  are  a  few 
other  points  (not  essential  to  the  efficiency  of  the  microscope) 
in  which  expense  is  avoided.  But  as  improvements  are  being 
constantly  introduced,  those  who  require  a  good  dissecting 
microscope  should  procure  the  catalogues  of  all  the  various 
dealsrs  before  they  decide. 


46  SELECTION  AND   USE 

Binocular  Dissecting  Microscope.— Tlie  binocular 
principle,  when  applied  to  the  dissecting  microscope,  is  of  the 
greatest  value,  as  it  enables  us  to  estimate  accurately  the  posi- 
tions, vertically,  cf  the  several  parts,  and  wo  can  thus  direct 
needles,  knives,  scissors,  etc.,  to  the  exact  point  that  we  wish  to 
operate  on  with  an  accuracy  that  is  impossible  with  the  monocu- 
lar dissecting  microscope.  Nachet  and  Beck  manufacture  very 
excellent  binocular  dissecting  microscopes,  and  the  only  thing 
to  be  regretted  is  that  the  price  is  necessarily  somewhat  high. 

PEINCIPAL  FORMS  OF   THE   COMPOUND 
MICROSCOPE. 

The  variety  of  models,  styles,  or  patterns  which  have  been 
devised  for  the  stands  and  for  the  general  arrangement  of  com- 
pound microscopes,  is  almost  infinite,  and  as  they  are  con- 
tinually changing,  it  would  be  a  hopeless  task  to  attempt  to 
give  a  description  of  all,  or  even  of  any  large  number  of  them. 
Fortunately,  from  motives  of  self-interest,  the  manufacturers  of 
these  instruments  promptly  publish  full  descriptions  and  en- 
gravings of  new  styles  as  soon  as  they  bring  them  out,  and 
therefore  the  best  plan  for  those  who  desire  to  make  a  judicious 
selection,  is  to  procure  the  catalogues  of  as  many  manufacturers 
as  possible,  and  carefully  compare  the  several  advantages  of  the 
different  forms.  The  addresses  of  all  the  prominent  manufac- 
turers are  published  from  year  to  year  in  the  "  Microscopist's 
Annual,"  a  cheap  little  volume  devoted  to  statistical  informa- 
tion of  this  kind. 

Cheap  Foreign  Stands.-— A  few  years  ago,  the  only 
stands  attainable  by  those  who  could  not  afford  an  expensive 
article,  were  the  cheap  French  and  German  microscopes. 
These  were  imported  in  large  numbers  by  dealers  in  optical 
goods,  and  were  distributed  to  different  parts  of  the  country 
through  the  agency  of  watchmakers,  jewellers,  booksellers,  and 
those  who  dealt  in  spectacles  and  opera  glasses,  and  conse- 
quently were  called  "opticians."  Fortunately  the  production 
of  really  good  and  cheap  American  microscopes  has  almost 
driven  this  class  of  instruments  out  of  market,  but  as  they  are 
still  to  be  found  on  sale,  a  few  words  in  regard  to  them  may  be 


OF  THE   MICBOSCOPE. 


necessary.  One  of  the  most  common  models  is  that  shown  in  Fig. 
12.  This  form,  although  modern  when  compared  with  the  micro- 
scopes of  Adams,  Baker,  etc..  is  one  of  the  oldest  in  nse.  It 
was,  we  believe,  devised  by  Wollaston,  as  a  stand  for  his 
doublet,  and  was  afterwards  modified  by  Fraunhofer,  whose 
microscopes  on  this  model  were  almost  identical  with  that 
shown  in  the  engraving.  It  is  now  too  well  known  to  need 
elaborate  description,  and  the  smaller  sizes  are  still  sold  ex- 
tensively for  the  use  of  young  people.  Being  manufactured  in 
large  quantities,  they  are  sold  very  cheaply,  when  the  quality 
of  the  lenses  is  taken  into  consideration.  Therefore,  until 
some  manufacturer  concentrates  his  efforts  upon  the  produc- 
tion of  the  more  convenient  forms,  and 
turns  them  out  in  very  large  numbers, 
the  vertical  microscope  will  probably 
maintain  its  place  in  the  market,  and 
many  beginners  will  be  led  into  buying 
an  instrument  which,  even  in  its  most 
complete  and  perfect  form,  will  almost 
certainly  be  a  source  of  dissatisfaction. 

Aside  from  any  deficiencies  which  may 
exist  in  the  optical  parts  of  the  instru- 
ment, the  objections  to  this  model  are  the 
small  size  and  inconvenient  form  of  tho 
stage  ;  the  fact  that  the  microscope  can  be. 
Used  only  in  the  vertical  position,  which, 
although  occasionally  very  useful,  is  tho 
worst  position  in  which  the  instrument 
can  be  placed  for  protracted  observations, 
and  the  arrangement  of  the  mirror,  which 
renders  the  proper  management  of  the  illu- 
mination impossible.  Therefore,  when- 
ever there  is  a  possibility  of  choice,  some 
form  other  than  the  vertical  should  be 
chosen. 

Occasionally  microscopes  of  this  kind 
are  furnished  with  achromatic  objectives  of 
pretty  fair  quality.  In  such  cases  the  ob- 
jectives and  eye-pieces,  if  they  could  be 


Fig.  12. 


VERTICAL  MICROSCOPE. 


48  SELECTION  AND   USE 

applied  to  a  better  stand,  would  be  worth  more  than  tlie  whole 
microscope  in  its  original  form. 

There  is,  however,  one  very  useful  purpose  to  which  cheap 
microscopes  of  this  kind  maybe  applied.  They  make  a  very 
convenient  and  efficient  pocket  microscope  for  collecting.  The 
stand  should  be  cut  off  above  the  mirror,  the  flat  stage  being 
left  with  enough  of  the  tube  adhering  to  it  to  give  it  stiffness. 
A  drop  of  water  being  placed  in  a  cell,  and  covered  with  thin 
glass,  is  laid  on  the  stage  and  held  there  with  the  fingers.  To 
illuminate  the  object  the  microscope  is  simply  held  up  to  the 
sky.  Those  who  object  to  holding  the  slide  .with  the  fingers, 
can  have  a  round  plate  of  metal  soldered  to  the  under  side  of 
the  stage,  so  as  to  make  it  even.  Movable  clips,  made  by 
bending  a  narrow  strip  of  hard  sheet  brass,  so  that  both  legs  are 
even,  can  then  be  slipped  on  so  as  press  on  the  slide,  and  on 
the  under  side  of  the  stage.  Such  a  microscope  is  very  light, 
and  may  be  readily  carried  in  a  tubular  pasteboard  case,  which 
any  one  can  make. 

Setting  aside  these  obsolete  forms,  as  well  as  those  models 
which  have  been  devised  for  special  purposes,  we  find  that  of 
the  stands  which  are  best  suited  to  the  purposes  of  the  phy- 
sician, the  naturalist,  the  student,  and  the  family,  there  are  four 
distinct  styles,  which  may  be  distinguished  as  follows  :  The 
Hoss,  the  Jackson,  the  Continental,  and  the  New  American 
models.  In  order  to  give  the  reader  an  idea  of  these  different 
models,  and  to  explain  the  advantages  and  disadvantages  of 
each,  we  give  engravings  and  such  descriptions  as  will  enable 
him  to  form  some  idea  of  the  stand  best  suited  to  his  special 
wants.  In  selecting  illustrations  of  the  different  types,  we  have" 
taken  the  cheaper  forms  in  preference  to  the  more  perfect,  but 
more  expensive  models  ;  and  as  our  object  is  to  describe  the 
genera]  features  of  the  stands  themselves,  and  not  to  detail  the 
merits  and  point  out  the  faults  of  those  parts  which  may  be 
easily  changed  or  altered,  we  have  omitted  all  description  of 
diaphragms,  eye-pieces,  objectives,  etc. 

Tlio  Ross  Model.— The  distinguishing  characteristic  of 
this  model  is  the  mode  in  which  the  body  is  supported.  By  re- 
ferring to  Plate  I,  it  will  be  seen  that  the  body  is  attached  at 


OP  THE  MICKOSCOPE.  49 

its  lower  end  to  a  transverse  arm,  which  in  turn  is  supported 
by  a  stout  bar,  which  is  moved  up  and  down  by  means  of  a 
rack  and  pinion.  This  movement  constitutes  the  coarse  ad- 
justment, the  fine  movement  being  effected  by  means  of  a  lever 
which  is  concealed  in  the  transverse  bar,  and  acts  upon  the 
nose-piece. 

So  far  as  mere  questions  of  convenience  and  adaptability  to 
different  kinds  of  work  is  concerned,  this  model  is  all  that  can 
be  desired,  and  as  made  by  Koss  &  Co.,  the  workmanship  is  so 
perfect,  and  the  finish  so  exquisite,  that  it  has  long  maintained 
a  high  position  in  public  favor.  It  has,  therefore,  had  numerous 
imitators,  and  has  probably  been  copied  more  extensively  than 
any  other  model  in  existence.  At  the  recent  Centennial  Exhibi- 
tion there  were  microscopes  on  this  model  from  the  most  widely 
scattered  localities.  Canada  was  represented  by  two  micro- 
scopes made  after  this  design.  Unfortunately,  however,  this 
model  is  one  of  the  very  worst  that  a  poor  workman  can 
attempt  to  imitate,  for  unless  the  workmanship  is  far  above  the 
average,  the  results  are  execrable.  The  reason  for*  this  is  very 
obvious.  The  body,  being  supported  only  at  the  lower  end, 
every  vibration  causes  the  upper  end  to  swing  through  a  com- 
paratively large  arc,  and  hence  any  motion  arising  from  loose- 
ness in  the  joints  is  multiplied  a  hundred  fold.  And  even  when 
the  joints  are  firm  and  without  shake,  any  vibration  communi- 
cated to  the  table  on  which  the  instrument  stands,  is  greatly 
increased  in  its  effects  when  it  reaches  the  upper  end  of 
the  body.  In  addition  to  this,  the  unsupported  part  of  the 
body  acquires,  by  each  movement,  a  momentum  which  reacts 
powerfully  on  the  lower  part,  and  consequently  on  the  ob- 
jective. 

These  defects  have  induced  Messrs.  Ross  &  Co.  to  bring  out 
a  new  pattern  designed  after  the  Jackson  model.  This  design 
has  been  carefully  worked  out  by  Mr.  "Wenham,  and  is  certainly 
very  beautiful  in  appearance,  and  very  efficient  and  convenient 
in  use.  Our  readers  will  therefore  bear  in  mind,  that  all  micro- 
scopes made  by  Eoss  &  Co.,  are  not  made  on  the  "Ross 
model." 


50  SELECTION  AND  tJSE 

The  Jackson  Model.  — The  special  characteristic  of  this 
model  lies  in  the  fact  that  the  "  ways  "  of  both  the  coarse  and 
fine  movements  are  brought  as  close  as  possible  to  the  optic  axis 
of  the  instrument.  In  the  Ross  model  the  coarse  movement  is  at 
a  considerable  distance  from  this  axis,  and  any  lateral  motion 
which  may  take  place  on  the  sliding  part  is  greatly  magnified 
at  the  axis  of  the  body — the  tram  verse  bar,  which  supports  the 
latter,  acting  like  the  long  arm  /)f  a  lever  to  greatly  multiply 
that  which  at  first  was  very  insignificant.  Hence  the  advan- 
tages which  the  Jackson  model  presents  are  great  steadiness  and 
the  fact  that,  in  common  with  the  Ross  model,  it  affords  abun- 
dant room  underneath  the  stage  for  those  accessory  methods  of 
illumination  which  are  indispensable  in  the  highest  class  of 
work.  It  is  almost  impossible  to  attach  to  the  smaller  patterns 
of  the  Continental  model  a  convenient  sub-stage,  carrying 
polarizer,  achromatic  condenser,  paraboloid,  etc.,  while  the 
model  under  consideration  is  specially  designed  to  receive 
these  important  accessories. 

This  model  is  a  very  general  favorite  both  with  English 
and  American  makers.  Amongst  the  latter  it  has  been  adopted 
as  the  best  for  all  first-class  stands.  The  engraving,  Plate  II, 
shows  a  very  excellent  form  of  this  model,  made  by  James  W. 
Queen  &  Co.  It  will  be  seen  that  along  a  great  part  of  its 
length  the  body  is  attached  to  the  rack-bar,  and  this  bar  slides 
on  long  ways,  which  give  great  steadiness  and  freedom  from 
vibration.  A  special  feature  of  this  stand  is  the  low  position 
of  the  milled  head  for  the  fine  adjustment.  This  enables  the 
microscopist  to  rest  the  hand  on  the  table  so  as  to  avoid  much 
of  that  slight  but  imperceptible  vibration  which  the  unsup- 
ported hand  is  apt  to  impart. 

Tlie  Continental  Form.— Most  of  the  stands  made  by 
the  better  class  of  French,  German,  and  Austrian  microscope 
makers  are  characterized  by  a  low,  compact  form,  and  great 
solidity  and  simplicity  of  construction.  They  are  intended  to 
be  used  chiefly  in  a  vertical  position,  and  hence  the  bodies 
are  short,  and  the  space  beneath  the  stage  is  contracted  to  the 
last  degree.  And  as  a  low  price  is  an  important  feature  in  these 
microscopes,  the  coarse  movement  is  generally  secured  by 


OF  THE   MICROSCOPE.  51 

simply  sliding  the  body  through  a  tube  or  collar.  In  the  higher- 
priced  stands  the  coarse  movement  is  effected  by  means  of  a 
rack  and  pinion,  arranged  as  in  the  "Jackson  "  model,  but  in 
both  styles  the  fine  movement  is  generally  arranged  in  the 
place  where  the  coarse  movement  is  located  in  the  "Ross" 
stand.  These  features  \vill  be  found  to  characterize  the  micro- 
scopes of  Chevalier,  Hartnack,  Xachet,  Zeiss,  and  others. 

The  term  "Continental"  model  has  been  applied  by  our- 
selves and  others  to  any  low,  compact  form  of  microscope,  but 
a  careful  study  of  the  subject  has  satisfied  us  that  this  is  not 
correct.  The  characters  above  given  are  peculiar  to  what 
ought  to  be  known  as  microscopes  of  the  Continental  model. 

The  Continental  model  has  some  advantages  on  the  score  of 
convenience,  and  as  large  numbers  of  our  medical  students 
have  been  educated  to  its  use,  it  has  been  such  a  favorite  that 
many  of  our  prominent  makers  have  imitated  it  to  a  certain 
degree.  For  the  ordinary  work  of  the  physician  and  the  his- 
tologist,  a  low  microscope,  which  may  be  easily  arranged  for 
work,  and  which  may  be  used  conveniently  in  a  vertical  posi- 
tion, is  certainly  desirable,  but  other  forms,  possessing  the 
same  advantages  and  without  the  objectionable  features  of  the 
Continental  model,  are  now  in  market,  and  have  taken  their 
place. 

The  objections  to  the  Continental  model  as  a  microscope  for 
the  higher  classes  of  work,  are  the  want  of  space  below  the 
stage  for  illuminating  accessories,  and  the  liability  of  the  fine 
movement  to  get  out  of  order.  For  it  is  obvious  that  a  very 
little  wear  on  the  "  ways  "  or  "slides  "  will  allow  the  body  to 
have  considerable  lateral  motion,  the  distance  of  the  "ways" 
from  the  optic  axis  of  the  instrument  being  very  great.  Some 
of  our  American  makers,  who  have  adopted  this  fine  motion, 
have  gone  so  far  as  to  use  hard  steel  pins  sliding  on  hardened 
ways  as  guides,  but  with  all  their  care  lateral  motion  ensues 
after  a  time. 

Tlie  New  American  Model.— The  more  general  em- 
ployment of  objectives  of  wide  angles  of  aperture,  and  of  im- 
proved methods  of  using  them,  have  led  American  manufac- 
turers to  introduce  certain  improvements  in  stands  of  moderate 


52  SELECTION    AND    TTSE 

price,  and  the  stands  which  embody  these  improvements  and 
modifications  are  so  distinct  that  they  form  a  class  by  them- 
selves, which  may  be  designated  the  ' '  New  American  Model. '' 
None  of  the  individual  features  found  in  this  niodel  are  new? 
the  most  important  of  them  having  been  embodied  many  years 
ago  in  the  English  patent  of  Grubb,  wrhile  others  were  used  long 
ago  by  Spencer  in  this  country.  But  Grubb's  invention,  so  far 
as  we  know,  never  came  into  use,  and  it  was  not  until  the  year 
1875,  that  the  importance  of  the  improvements  in  question  be- 
came generally  recognized.  Towards  the  close  of  that  year,  and 
the  beginning  of  1876,  four  of  the  prominent  microscope  makers 
of  this  country,  viz.,  Bulloch,  Gundlach,  Tolles,  and  Zentmayer 
seem  to  have  turned  their  attention  to  the  subject,  and  shortly 
after  brought  out  models  on  the  new  plan,  the  distinctive  fea- 
tures of  which  are  the  swinging  of  the  mirror  around  a  centre 
which  lies  in  the  plane  of  the  object  ;  the  combination  of  a 
swinging  with  a  longitudinal  movement  in  the  sub-stage,  and 
the  use  of  a  very  thin  stage.  Prior  to  this  time,  in  all  the  best 
microscopes  of  English  and  American  make  (at  least  as  de- 
scribed in  the  catalogues  of  the  dealers),  the  sub-stage  was  made 
to  move  only  in  the  line  of  the  optic  axis  of  the  instrument ; 
the  American  makers  took  a  new  departure,  and  the  new  model 
is  the  result.  In  its  popular  forms  this  model  is  characterized 
by  great  simplicity  in  its  working  parts,  while  at  the  same  time, 
being  provided  with  the  very  best  means  for  adjusting  and 
registering  the  illumination,  it  is  a  stand  which  is  sufficient  for 
the  highest  class  of  work,  except,  of  course,  in  a  few  special 
departments. 

So  far  as  our  knowledge  goes  there  are  at  present  before  the 
public  four  stands  of  comparatively  low  price,  which  may  be 
relegated  to  this  class.  They  are:  The  "Acme,"  of  J.  W.  Queen 
&  Co.  ;  the  "Biological"  of  Bulloch  ;  the  "  Histological  "  of 
Zentmayer,  and  the  "Investigator''  of  the  Bausch  &  Lomb 
Optical  Company.  The  large  stands  of  the  Bausch  &  Lomb 
Optical  Co.,  of  Beck,  Bulloch,  Eoss,  Tolles,  Zentmayer,  and 
others,  are  of  course  quite  as  efficient  as  the  small  stands  we 
he/ve  named,  but  the  price  is  greater,  and  the  convenience  of 
the  small  stands,  when  used  as  working  microscopes,  is  in  their 
favor. 


OF  THE  MICROSCOPE.  53 

This  principle  of  arranging  the  various  parts  to  revolve  round 
the  object  as  a  center,  as  it  were,  was  carried  out  in  the  most 
complete  manner  by  Mr.  "Wenham  in  his  concentric  model,  a 
simplified  form  of  which,  as  made  by  the  Bausch  &  Lomb  Op- 
tical Co.,  is  shown  in  Plate  VII. 

;t  The  Biological  Stand  of  Mr.  Bullock  is  shown  in  Plate  III. 
It  will  be  seen  at  once  that  this  stand  is  substantial  and  firm 
in  all  its  parts.  The  coarse  movement  is  by  rack  and  pinion, 
and  the  body  is  supported,  as  in  the  Jackson  model,  along 
nearly  its  entire  length.  The  fine  movement  acts  upon  the 
entire  body,  the  coarse  movement  included,  so  that  the  distance 
between  the  eye-piece  and  the  objective  is  not  affected  by  it. 
The  stage  is  thin,  but  substantial,  and  is  so  cut  away  at  the 
well-hole  that  light  of  great  obliquity  can  be  used.  The  stage- 
plate  rotates  in  a  ring,  and  may  be  clamped  when  necessary  by 
means  of  the  small  screw  seen  in  front.  By  loosening  this 
screw  the  plate  may  be  entirely  removed,  and  then,  when  the 
body  stands  upright,  a  simple  plate  of  glass  may  be  used  for  a 
stage,  and  thus  all  danger  of  injury  from  acids  is  avoided.  The 
ring  in  which  the  stage-plate  rotates  is  held  in  place  by  two 
capstan-headed  screws,  and  when  these  are  slacked  it  is  easy 
to  adjust  the  stage  so  that  it  shall  be  concentric  with  the  optic 
axis.  The  sub-stage  is  arranged  for  adjustment  in  the  same 
way.  It  is  adapted  to  carry  the  standard  size  accessories,  and 
is  furnished  with  an  adapter  which  has  the  Society  screw,  so 
that  ordinary  objectives  may  be  used  as  condensers.  Mr. 
Bullock  also  furnishes  a  diaphragm  of  peculiar  form  (shown  in 
the  engraving),  which  we  have  found  very  efficient,  both  with 
direct  and  oblique  light.  The  special  feature  of  models  of  this 
class  is,  however,  shown  in  the  method  of  hanging  the  mirror 
and  sub-stage.  These  are  attached  to  separate  bars,  which 
rotate  on  an  axis,  the  plane  of  which  is  a  little  above  the  level 
of  the  stage.  A  stop  is  also  provided,  whereby  they  may  both 
be  placed  precisely  in  the  line  of  the  optic  axis.  Both  mirror 
and  sub-stage  may  be  moved  towards  the  stage  and  from  it,  and 
the  movements  of  both  are  entirely  independent.  The  conse- 
quence is  that  the  various  forms  of  illuminating  apparatus  have 
a  range  of  adjustment  that  cannot  be  obtained  with  the  older 
models. 


54  SELECTION   AND   USE 

The  instrument,  as  shown  in  the  cut,  is  about  two-fifths  of 
the  real  size.  When  the  draw-tube  is  out,  the  body  has  the 
standard  length  of  ten  inches,  and  it  is  furnished  with  the 
"  broad-gauge  "  screw,  to  which  is  fitted  an  adapter  carrying 
the  standard  Society  screw. 

The  first  stand  placed  in  market  at  a  moderate  price,  and  em- 
bodying the  special  features  of  the  new  American  model,  was 
the  Histological  stand  of  Mr.  Zentmayer,  which  is  shown  in 
Plate  IV.  The  base  and  uprights  of  this  stand  are  one  piece, 
of  a  peculiar  shape,  of  great  rigidity,  to  which  the  bell-metal 
bar  is  attached  by  a  joint,  allowing  the  use  of  the  instrument  at 
any  angle  of  inclination  ;  the  perpendicular  and  horizontal  po- 
sitions are  indicated  by  stops.  The  coarse  adjustment  is  by  a 
rack  and  pinion,  or  in  cheaper  forms  by  a  sliding  tube. 

The  fine  adjustment  is  by  a  concealed  lever,  acted  upon  by  a 
delicate  screw,  and  moving  the  entire  body,  which  is  fitted 
to  the  grooved  bar,  giving  a  steady  and  delicate  movement. 
The  arrangement  of  the  swinging  sub-stage  and  mirrors  is 
easily  understood  from  the  engraving. 

The  removable  sub-stage  carries  the  diaphragms,  which  can 
be  shifted  up  close  to  the  object. 

The  stage  is  a  modification  of  the  glass  stage,  and  consists  of 
a  glass  bar  kept  down  by  two  spring  clips,  against  which  the 
object  rests.  By  this  method  the  object  may  be  moved  in  the 
latitude,  the  longitudinal  movement  being  accomplished  by 
hand.  The  spring  clips  may  be  used  independently  for  hold- 
ing anything  in  a  fixed  position,  by  simply  placing  them  in  the 
extra  holes  provided  for  that  purpose. 

The  ''Investigator"  stand  is  shown  in  Plate  V,  and  also 
presents  the  special  features  which  we  have  mentioned.  The 
mirror-bar  is  swung  so  that  its  axis  of  motion  lies  in  the  plane 
of  the  object,  and  it  carries  the  sub-stage,  which  may  thus  be 
inclined  to  any  angle  with  the  plane  of  the  stage,  or  it  may  be 
brought  entirely  above  the  latter.  The  mirror  is  attached  to 
the  mirror-bar  by  means  of  a  secondary  bar,  which  thus  permits 
the  use  of  the  mirror  independently  of  the  sub-stage,  and  both 
mirror  and  sub-stage  may  be  moved  to  and  from  the  object. 
The  sub-stage  may  be  removed  entirely  when  desirable. 

In  order  to  secure  the  convenience  of  the  low  Continental 


OF   THE   MICROSCOPE.  55 

model,  this  microscope  is  provided  with  two  draw-tubes,  sliding 
one  within  the  other,  so  that  the  body  may  be  shortened  as 
much  as  possible  when  desired.  The  first  draw-tube  has  the 
Society  screw  at  the  lower  end,  so  that  low-power  objectives  of 
very  great  working  distance  may  be  used  ;  and  as,  by  means  of 
the  second  draw,  the  draw-tube  may  be  made  of  the  usual 
standard  length  (ten  inches),  it  may  be  used  as  the  body  for 
any  objective,  and  quickly  focussed  by  sliding  it  in  the  outer 
tube,  the  same  as  in  instruments  without  rack  and  pinion  ad- 
justment. The  "  broad-gauge  "  screw  has  been  added,  so  that 
low-power  objectives  of  wide  angle  may  be  used  to  the  greatest 
advantage. 

Cheaper  Stands.— The  stands  which  we  have  just  de- 
scribed, are,  of  necessity,  somewhat  expensive,  because,  unless 
the  workmanship  is  of  the  very  best,  the  performance  is  quite 
unsatisfactory  ;  and  as  a  stand  alone  is  of  no  use.  an  outfit 
which  includes  such  a  stand  is  frequently  beyond  the  means  of 
students  and  others.  To  meet  the  wants  of  such  persons,  the 
various  manufacturers  we  have  named,  together  with  Klein, 
Pike,  Schrauer,  and  others,  have  brought  out  stands,  which, 
when  complete,  with  case  and  objectives,  are  sold  for  about 
the  price  of  the  stands  we  have  Described.  For  the  study  of 
elementary  botany,  histology,  etc.,  many  of  these  cheap  micro- 
scopes answer  a  very  good  purpose.  They  enable  us  to  follow 
the  descriptions  given  in  the  books,  and  the  view  of  any  object 
which  we  obtain  from  even  the  poorest  of  them,  is  infinitely 
better  than  any  representation  which  can  be  given  by  an  en- 
graving. The  better  class  of  these  microscopes  answer  well  for 
the  ordinary  purposes  of  the  physician,  and,  as  they  are  simple 
and  easily  used,  they  are  frequently  preferred  to  more  expensive 
and  more  complicated  instruments. 

Tlie  Binocular  Microscope.— More  than  two  hundred 
years  ago,  attempts  were  made  to  construct  binocular  micro- 
scopes, and  yet  a  good  and  efficient  binocular  is  a  thing  of 
yesterday.  The  first  really  efficient  binocular  microscope  was 
constructed  by  Prof.  Eiddell,  of  New  Orleans,  about  the  year 
1853,  and  to  this  fact  Mr.  Wenham  has  borne  free  and  generous 


56  SELECTION  AND  USE 

testimony  in  a  letter  addressed  to  the  English  Mechanic.  To 
Mr.  Wenham,  himself,  however,  is  due  the  honor  of  having 
devised  a  form  of  the  binocular  which  is  at  once  efficient 
and  of  moderate  price.  Mr.  Wenliam's  form  has  been  adopted 
bj  all  the  American  and  English  makers  that  manufacture  bi- 
noculars, and  he  has.  therefore,  laid  all  microscopists  under 
deep  obligations,  not  only  by  devising  such  simple  and  efficient 
means  of  accomplishing  a  most  desirable  result,  but  by  giving 
the  use  of  his  invention  freely  to  the  world. 

Of  the  value  of  the  binocular,  there  is  a  wide  difference  of 
opinion,  some  regarding  it  as  a  mere  toy,  altogether  beneath 
notice  as  an  instrument  of  scientific  research,  while  others  con- 
sider it  a  most  important  addition  to  our  means  of  investiga- 
tion. Since,  however,  it  will  almost  always  be  found  that  those 
who  place  a  high  value  on  the  binocular  are  those  who  have 
used  it  most,  while  those  who  decry  it  know  absolutely  nothing 
of  its  merits,  and  in  some  cases  are  even  ignorant  of  the  man- 
ner of  using  it,  the  reader  will  have  but  little  difficulty  in  de- 
ciding on  which  side  the  truth  lies.  In  England,  where  cheap 
and  good  binoculars  are  common,  this  form  of  instrument  has 
become  a  great  favorite  with  all  the  noted  microscopists,  and 
•we  refer  not  merely  to  men  who  own  microscopes  as  a  means  of 
amusement,  but  to  those,  who,  like  Carpenter  and  others,  have 
enlarged  the  boundaries  of  knowledge  by  their  researches.  At 
present  the  binocular  is  gaining  rapidly  in  favor  in  this  country, 
and  very  excellent  instruments  are  now  turned  out  by  several 
of  our  American  microscope  makers. 

There  are,  of  course,  certain  limits  to  the  range  of  usefulness 
of  the  binocular  microscope.  As  at  present  constructed,  it  is 
most  efficient  in  the  use  of  the  low  powers,  and  hence,  there 
are  certain  classes  of  work  to  which  it  is  peculiarly  applicable, 
while  in  other  branches,  particularly  certain  departments  of 
histology,  it  is  of  comparatively  slight  advantage.  And  it  will 
in  general  be  found  that  the  principal  opposition  to  the  binoc- 
ular has  come  from  continental  histologists,  who,  because  they 
found  it  of  little  use  to  themselves,  concluded  that  it  could  not 
possibly  be  of  use  to  any  one  else. 

The  advantages  presented  by  binocular  instruments  are  two- 
fold ;  the  relief  to  the  observer  arising  from  the  ability  to  use 


OF   THE   MICROSCOPE.  57 

both  eyes  is  very  great,  and  the  view  which  is  obtained  of  any  ob- 
ject is  so  much  clearer  and  more  realistic,  that  we  at  once  per- 
ceive, by  our  .mere  sense  of  vision,  those  features  which  we  would 
otherwise  have  to  work  out  by  tedious  mental  processes.  It  has 
been  said  by  some  that  the  binocular  is  apt  to  exaggerate  the 
stereoscopic  effects,  and  give  false  views.  This  is  certainly  not 
the  case  under  ordinary  circumstances,  and  we  doubt  much  if 
it  ever  occurs. 

"We  have  selected  as  an  illustration  of  the  binocular  micro- 
scope, one  of  moderate  cost,  which   combines  the   binocular 
feature  with  those  peculiar  to  the  new  American  model — the 
United  States  Army  Hospital  Stand,  made  by  Zeutmayer  of 
Philadelphia.     The  details  of  this  stand  have  been  worked  out 
with  special  reference  to  the  use  of  objectives  of  high  angles. 
The  substage,  which  carries  the  accessories  and  the  plane  and 
concave  mirrors,  swings  in  the  same  manner  as  in  the  well- 
known  American  Centennial  Stand  of  the  same  maker,  having 
the  object  as  its  center,  even  when  swung  over  the  stage  for 
illuminating  opaque  objects,   thus  making  a  bull's-eye  con- 
denser unnecessary.     It  is  provided  with  a  graduated  circle, 
not  shown   in  the  engraving,   for  registering   the  degree  of 
obliquity,  and  a  stop  to  indicate  when  it  is  central  with  the 
main  body. 

The  binocular  in  general  use,  being  that  in  which  the  Wen- 
ham  prism  is  used,  consists  of  a  main  body  through  which  the 
half  of  the  rays  transmitted  by  the  objective  pass  directly  to  the 
eye-piece.  In  the  figure  (Plate  VI)  this  body  is  the  one  nearest 
to  the  reader,  and  connected  with  it  is  another  body,  through 
which  the  other  half  of  the  rays  pass — they  being  diverted  in 
this  direction  by  the  prism,  which,  when  in  place,  cuts  the  cone 
of  rays  from  the  objective  in  two.  The  frame  which  carries 
this  prism  is  seen  just  above  the  point  at  which  the  objective  is 
attached,  and  it  is  so  arranged,  that  when  desirable  it  may  be 
drawn  out,  and  thus  the  prism  becomes  inoperative,  and  the  in- 
strument is  practically  converted  into  a  monocular  one.  This 
is  one  of  the  important  advantages  which  are  peculiar  to  the 
Wenharn  prism.  In  some  recent  forms  of  the  binocular  the 
entire  fittings  of  the  prism  may  be  removed,  so  as  not  to  inter- 
fere with  the  wide-angle  lenses  of  low  power. 


58  SELECTION   AND    USE 

As  eyes  differ  greatly  in  their  distance  apart,  the  eye-pieces 
in  the  binocular  are  made  variable  in  this  respect  by  being 
pushed  out  or  in,  either  by  a  lever  or  a  rack  and  pinion.  As 
the  bodies  are  not  parallel,  but  form  an  angle  with  each  other, 
it  is  evident  that  the  further  the  eye-pieces  are  drawn  out  the 
wider  apart  do  they  become. 

Binocular  Eye-Piece.— A  very  valuable  and  efficient 
means  of  converting  an  ordinary  monocular  microscope  into  a 
binocular,  has  been  devised  by  Mr.  Tolles.  The  one  which  we 
possess  gives  excellent  results  with  objectives  of  higher  power 
than  those  available  with  the  Wenham  prism  as  ordinarily  con- 
structed, and  it  is  greatly  to  be  regretted  that  its  high  price 
($80)  places  it  beyond  the  reach  of  those  for  whom  this  work 
is  chiefly  intended.  It  has  been  copied  extensively  in  Europe, 
but  we  believe  that  it  is  now  definitely  settled  that  the  honor  of 
the  invention  belongs  to  Mr.  Tolles. 

The  Inverted  Microscope.— Some  years  ago,  Professor 
J.  Lawrence  Smith  devised  a  microscope  specially  adapted  to 
chemical  investigations.  In  this  instrument  the  stage  is  placed 
over  the  objective,  which  is  inverted,  and  the  rays  from  the 
object,  after  being  reflected  down  through  the  objective,  enter 
a  prism,  which  so  changes  their  direction  that  they  pass  up  the 
body  to  the  eye-piece,  and  thence  to  the  eye  of  the  observer. 
For  very  many  investigations  the  inverted  microscope  is  a  very 
useful  instrument. 

Lithological  Microscopes. — The  application  of  the 
microscope  to  petrology,  or  the  study  of  rocks,  has  now  become 
such  an  important  department  of  scientific  research  that  micro- 
scopes specially  adapted  to  this  purpose  are  manufactured  by 
several  makers.  These  microscopes  possess  several  very  im- 
portant conveniences,  which  are  almost  indispensable  for  such 
work,  but  which  the  limits  of  the  present  volume  forbid  us  to 
describe  at  length. 

The  Aquarium  Microscope.— The  importance  of  the 
compound  microscope  in  the  examination  of  small  portions  (in 
situ]  of  large  objects,  without  any  preparation  whatever,  has  led 


OF   THE   MICROSCOPE.  59 

to  the  construction  of  microscopes  consisting  simply  of  a  body 
with  its  adjustments  for  focussing  arranged  on  the  simplest 
kind  of  a  support — usually  a  rod  and  foot  somewhat  like  a  re- 
tort stand.  Such  microscopes  have  been  used  largely  for  exam- 
ining objects^  in  aquaria,  and  hence  the  name.  They  are  also 
applicable  to  the  examination  of  writing,  the  sheet  of  paper 
being  simply  spread  out  on  a  table  and  the  microscope  placed 
on  it  ;  to  the  examination  of  portions  of  the  surface  of  the 
human  body  in  various  cases  of  skin  disease,  and,  in  short,  to 
the  examination  of  any  object  which  cannot  be  conveniently 
placed  on  the  stage  of  an  ordinary  microscope. 

Microscopes  for  Special  Purposes.— In  addition  to 
the  microscopes  which  we  have  described,  some  of  which  have 
been  devised  with  a  special  view  to  certain  particular  lines  of 
research,  it  often  happens  that  a  microscope  is  needed  for  a 
single  purpose,  and  for  no  other.  Thus,  for  example,  in  Ger- 
many, a  few  years  ago,  microscopes  were  made  and  sold  for  the 
sole  purpose  of  examining  flesh  for  trichinae.  These  micro- 
scopes were  very  simple,  and  when  their  quality  was  considered, 
they  were  very  cheap — the  cheapness  being  due,  in  a  great 
degree,  to  the  fact  that  no  provision  had  been  made  for  a 
multiplicity  of  conveniences,  which,  in  the  case  of  these  special 
instruments,  would  never  be  of  any  use.  As  the  microscope 
becomes  more  generally  used,  and  the  people  at  large  become 
more  thoroughly  acquainted  with  the  extraordinary  aid  which 
it  is  capable  of  rendering  to  those  who  are  engaged  in  the  arts, 
it  is  probable  that  the  numbers  of  these  cheap  microscopes  will 
increase,  in  which  case  we  may  expect  to  see  the  price  greatly 
reduced  in  proportion  to  the  quality. 

"Class"  Microscopes.— When  a  number  of  objects  are 
to  be  shown  to  a  class  or  to  a  scientific  meeting,  it  is  desirable 
to  avoid  the  inconvenience  arising  from  each  individual  having 
to  go  to  the  instrument,  and  this  is  unavoidable  where  ordinary 
microscopes  are  used  on  fixed  tables.  Prof.  Quekett  got  over 
this  difficulty  by  having  his  class-room  or  theatre  fitted  up  with 
rails  upon  which  the  microscopes  with  their  lamps  were  passed 
from  one  to  another,  and  where  the  company  is  small,  the  com- 
mon revolving  table  meets  every  requirement.  But  where  the 


60  SELECTION   AND   USE 

class  or  meeting  is  large,  the  best  device  is  that  of  Prof.  O.  W. 
Holmes,  whose  class  microscope,  as  made  by  Messrs.  J.  W. 
Queen  &  Co.,  is  shown  in  Figure  13.  This  instrument  is  of  the 
simplest  construction,  so  as  to  reduce  the  cost  as  much  as 
possible,  and  it  is  made  largely  of  wood,  so  as  to  be  light  and 
easily  held  in  the  hand.  The  objects  are  viewed  by  direct  light, 
without  the  intervention  of  a  mirror.  The  lamp  used  is  a  flat- 
wicked  one,  the  edge  of  the  wick  being  turned  towards  the 


Fig.  13. — HOLMES'  CLASS  MICROSCOPE. 


object  when  high  powers  are  used,  and  the  flat  side  of  the  wick 
when  the  powers  are  IOAV. 

"  For  opaque  objects,  the  lamp  is  raised  to  the  top  of  its  stand, 
and  its  rays  allowed  to  fall  upon  a  small  concave  mirror  at- 
tacked  to  the  iron  frame  by  a  universal  joint,  whence  they  are 
reflected  upon  the  object.  The  coarse  adjustment  is  effected  by 
sliding  the  body  through  a  short  split  tube,  and  the  fine  move- 
ment by  means  of  a  delicate  screw,  which  acts  on  the  stage. 
The  entire  height  of  the  instrument  is  about  twelve  inches,  size 
of  base  twelve  by  four  inches. 


OF  THE   MICROSCOPE.  61 


OBJECTIVES. 

The  modern  compound  microscope  owes  almost  all  its  value 
to  the  high  degree  of  perfection  which  has  been  attained  in  the 
construction  of  the  objectives  used  with  it.  Some  of  the  old 
microscope  stands  were  quite  as  elaborate,  and  quite  as  costly, 
as  anything  that  can  be  found  in  the  workshops  of  our  modern 
opticians,  out  from  the  fact  that  the  objectives  were  defective, 
their  value  as  instruments  of  research  was  of  a  very  low  degree. 
In  these  old  microscopes  even  the  highest  power  objectives  were 
simply  bits  of  ground  glass,  such  as  with  our  modern  appliances 
might  easily  be  produced  for  ten  cents  apiece,  while  a  modern 
objective  of  the  same  power,  by  any  of  our  first-class  makers, 
would  cost  at  least  one  hundred  dollars.  In  mechanical  exe- 
cution the  modern  objective  approaches  more  nearly  to  theo- 
retical perfection  than  any  other  instrument  made  by  man,  but 
the  reader  must  not  infer  from  this,  as  some  have  done,  that 
it  has  reached  the  limit  of  perfection,  and  that  nothing  more 
is  to  be  expected.  This  was  said  ten,  twenty,  and  thirty  years 
ago,  and  in  each  case  the  words  had  hardly  been  committed 
to  paper  before  some  optician  proved  their  falsity  by  accom- 
plishing that  which  had  just  been  declared  to  be  impossible  ! 
Within  the  past  few  years  the  most  wonderful  improvements 
have  been  effected,  and  from  present  indications  it  would  seem 
that  we  are  on  the  eve  of  still  more  startling  developments. 

Since,  then,  the  objective  is  really  the  most  important  part  of 
every  microscope,  seeing  that  the  quality  and  trustworthiness 
of  the  work  done  must  depend  to  a  very  great  extent  upon  its 
efficiency,  it  is  of  some  importance  to  the  microscopist  that  he 
should  have  at  least  a  fair  understanding  of  the  causes  to  which 
the  superiority  of  modern  objectives  is  due. 

When  we  use  a  simple  glass  lens  as  an  objective  for  a  com- 
pound microscope,  we  find,  on  attempting  to  examine  objects 
under  powers  of  more  than  one  hundred  diameters,  the  following 
defects  and  difficulties  :  The  field  of  view  is  so  dimly  illuminated 
that  objects  are  seen  with  difficulty  ;  the  outlines  of  the  different 
parts,  instead  of  being  sharp  and  clear,  are  thick  and  hazy ; 
several  of  the  lines  are  fringed  with  brilliant  colors,  but  colors 


62  SELECTION  AND    TTSE 

which  do  not  belong  to  the  objec's,  and  finally,  if  the  outlines 
of  the  object  should  happen  to  be  straight  lines,  and  be  known 
to  be  such,  ifc  will  ba  found  that  they  will  appear  to  be  curved 
and  distorted.  It  is  evident,  therefore,  that  a  simple  lens  can- 
not be  used  as  an  objective  in  any  important  work;  its  indica- 
tions are  unreliable,  and  the  imagination  is  allowed  full  scope, 
so  that  the  eye  is  enabled  to  see  whatever  the  mind  desires 
to  see. 

The  defects  which  we  have  just  detailed,  and  which  are  found 
in  every  simple  glass  lens,  whose  surfaces  are  bounded  with 
curves  that  are  parts  of  circles,  are  largely  due  to  what  is  called 
spherical  and  chromatic  aberration.  As  these  terms  are  probably 
not  familiar  to  many  of  our  readers,  we  will  give  as  full  and 
simple  an  explanation  of  the  subject  as  can  be  done  without 
the  formal  aid  of  mathematics. 

Spherical  Aberration.— The  enlarged  image  formed  at 
the  focus  of  any  lens,  and  rendered  visible  on  a  screen  or  sheet, 
is  produced  in  this  way:  The  rays  proceeding  from  the  object, 
and  passing  through  the  lens,  are,  by  the  action  of  the  glass, 
bent  from  the  path  they  would  otherwise  pursue.  The  object 
may  of  course  be  supposed  to  consist  of  an  infinite  number  of 
points,  and  from  these  points  rays  proceed  in  every  direc- 
tion, and  consequently  through  every  part  of  the  lens.  If  the 
lens  were  perfect,  all  the  rays  from  any  one  point  would  bs 
brought  together  at  a  second  point  corresponding  with  the 
first.  Unfortunately,  however,  the  ordinary  lens  does  not  do 
this;  the  central  portions  of  the  lens  and  the  outer  por- 
tions act  differently;  the  one  brings  the  rays  to  a  focus  at 
a  point  a  little  nearer  to  the  lens  than  the  other,  and,  con- 
sequently, although  we  move  the  screen  to  a  slightly  greater 
or  less  distance,  we  still  get  an  image  of  about  the  same 
degree  of  distinctness.  It  is  obvious,  therefore,  that  when 
placed  at  any  distance  within  certain  limits,  the  screen  will 
receive  not  one  image,  but  a  series  of  layers  of  images  as  it 
were,  and  this  consequently  gives  an  indistinctness  to  the 
resulting  image. 

Our  readers  will  find  no  difficulty  in  thinking  out  this 
matter  for  themselves,  and  when  they  have  arrived  at  clear 


OF  THE  MICROSCOPE.  63 

ideas  upon  the  subject,  they  will  see  that  spherical  aber- 
ration is  caused  by  the  difference  between  the  extent  of  the 
refraction  produced  at  different  parts  of  the  lens,  and  this 
applies  not  only  to  all  the  rays  proceeding  from  each  in- 
dividual point,  but  to  the  several  pencils  which  proceed  from 
different  points. 

It  is  evident  that  if  some  parts  of  the  lens  bring  the  rays  to  a 
focus  at  a  shorter  distance  than  others,  these  parts  must  magnify 
more,  and  such  is  in  reality  found  to  be  the  case.  But  if  one 
part  of  an  object  is  magnified  more  than  another,  the  image 
will  be  distorted,  and  hence  we  have  what  is  sometimes  known  as 
aberration  of  form.  This  distortion  is  easily  seen  by  examining  a 
piece  of  muslin  with  a  magnifier  of  high  power  and  large 
diameter.  The  threads  in  the  centre  of  the  field  of  view*  will 
appear  to  be  straight,  while  those  at  the  outside  will  appear  to 
be  curved,  f 

Chromatic  Aberration.  —  This  is  a  defect  of  ordinary 
or  unconnected  lenses,  whereby  they  not  only  act  as  magnifiers, 
but  as  prisms,  decomposing  the  light,  and  causing  objects  seen 
through  them  to  appear  with  a  fringe  of  color.  Common  hand 
magnifying  glasses,  used  in  the  ordinary  way,  do  not  exhibit 
this  defect  to  a  very  marked  degree,  but  when  the  images  formed 
by  lenses  of  this  kind  are  again  magnified,  as  is  done  in  the 
compound  microscope  or  telescope,  the  color  becomes  very  dis- 
agreeably perceptible. 

*  By  field  of  view  is  meant  that  portion  of  the  object  which  is  visible 
through  the  magnifier. 

fin  ordinary  lenses  and  microscopes,  in  which  this  defect  is  not  cor- 
rected by  the  structure  of  the  glasses  themselves,  the  effects  of  spherical 
aberration  are  lessened  by  contracting  the  field  of  view,  so  that  only  those 
parts  of  the  object  which  are  seen  through  the  centre  of  the  lens  or  objec- 
tive are  looked  at.  This  contraction  is  usually  effected  by  means  of  dia- 
phragms, or  round  plates  of  metal  pierced  with  a  central  hole,  which  are 
so  placed  as  to  cut  off  the  rays  which  pass  through  the  edge  of  the  lens, 
and  leave  only  those  that  are  central.  This  plan,  however,  is  only  the 
substitution  of  one  defect  for  another,  for  by  lessening  the  field  of  view  of 
the  lens,  we  are  prevented  from  seeing  more  than  a  very  small  portion  of 
the  object,  and  in  addition  to  this  ihe  light  is  so  much  reduced  that  the 
object  is  seen  only  with  very  great  difficulty,  and  not  at  all  clearly. 


64  SELECTION  AND    USE 

Corrected  Objectives.— The  defects  which  we  have  just 
described  have  been  the  chief  difficulties  in  the  way  of  perfect- 
ing both  the  microscope  and  the  telescope.  In  the  case  of  tlie 
latter,  however,  it  was  long  ago  found  that  very  excellent  re- 
sults could  be  obtained  by  forming  the  lenses  of  two  or  more 
pieces  of  glass  of  different  kinds,  and  numerous  attempts  were 
made  to  apply  the  same  principles  to  the  construction  of  the 
microscope,  but  without  marked  success.  The  small  lenses 
used  for  the  microscope  seemed  to  defy  the  skill  of  the  practi- 
cal opticians  of  those  days,  and  resort  was  had  to  such  devices 
as  lenses  made  of  precious  stones,  and  the  use  of  light  which 
could  not  be  decomposed — motto-chromatic  light  as  it  was 
called,  or  light  of  one  color.  Such  light  is  readily  procured  by 
the  combustion  of  alcohol  mixed  with  common  salt,  and  when 
illuminated  solely  by  such  a  light,  a  brilliantly  colored  painting 
looks  exactly  like  a  plain  black  and  white  engraving.  But 
although  the  use  of  such  a  light  lessens  the  evils  caused  by 
chromatic  aberration,  they  introduce  another  which  is  quite 
serious — objects  which  are  really  colored,  appear  in  black  and 
white  only.  Moreover,  such  a  light  cannot  easily  be  obtained 
of  a  brilliancy  sufficient  to  afford  good  illumination,  and  in 
addition  to  this  all  the  defects  due  to  spherical  aberration  still 
remain  in  full  force. 

The  first  attempts  made  to  perfect  the  object-glasses  of  micro- 
scopes, consisted  in  the  use  of  doublets  and  triplets,  it  having 
been  found  that  the  spherical  aberration  is  greatly  lessened, 
when  the  total  refraction  is  divided  up  amongst  several  surfaces 
of  moderate  curvature,  instead  of  one  surface  in  which  the  curva- 
ture is  excessive,  and  this  plan  is  still  pursued  in  the  construc- 
tion of  what  are  known  as  French  triplets,  which  will  be  de- 
scribed hereafter.  About  the  year  1829,  however,  Mr.  J.  J. 
Lister,  of  London,  England,  published  an  elaborate  paper 
upon  the  subject,  and  it  was  from  the  principles  laid  down  in 
this  paper,  that  all  the  important  improvements  in  the  modern 
objective  took  their  rise.  These  principles  were  embodied  in 
the  practical  construction  of  objectives  by  Andrew  Boss,  who 
suggested  the  important  improvement  known  as  the  adjust- 
ment for  thickness  of  cover.  To  Lister  and  Ross,  therefore, 
it  maybe  justly  said  that  we  owe  that  optical  wonder,  the 


OP  THE  MICROSCOPE.  65 

modern  objective,  for  although  great  improvements  have  been 
made  within  the  past  few  years,  it  is  upon  the  results  of  their 
labors  that  these  improvements  have  been  based.  And  yet, 
notwithstanding  this  well-known  fact,  the  names  of  these 
distinguished  microscopists  are  not  so  much  as  mentioned 
in  this  connection  in  the  recent  work  of  Dr.  Frey,  which  has 
been  lately  translated  into  English,  and  extensively  circulated 
in  this  country  ! 

In  estimating  the  quality  of  an  objective,  there  are  certain 
features  to  which  especial  attention  must  be  given.  Aside 
from  magnifying  power,  which,  of  cjurse,  cannot  be  regarded  as 
affecting  the  quality  of  an  objective,  these  points  are:  1.  Defin- 
ing power;  2.  Achromatism;  3.  Freedom  from  aberration 
of  form;  4.  Flatness  of  fie'd;  5.  Angular  aperture;  6.  Pene- 
tration; 7.  Working  distance.* 

Defining  Power.— This  is  undoubtedly  the  most  impor- 
tant quality  to  be  sought  for  in  objectives.  A  glass  that  is 
deficient  in  this  point  is  absolutely  worthless.  Want  of  de- 
fining power  is  shown  by  a  general  haziness  and  thickening  of 
the  outlines,  together  with  a  want  of  clearness  in  the  details. 
It  arises  from  the  presence  of  either  spherical  or  chromatic 

*The  authors  of  the  Mierographic  Dictionary  enumerate  the  following 
points  as  those  in  which  object-glasses  differ  from  each  other:  1.  Magni- 
fying power.  2.  Defining  power.  3.  Penetrating  power.  4.  Their  cor- 
rective adaptations.  The  functions  attributed  to  "  defining  po\vtr"are 
the  same  as  those  given  by  other  writers;  "penetrating  power  "-seems  to 
be  equivalent  to  what  is  generally  called  "resolving  power;"  "corrective 
adaptation"  is  merely  the  presence  of  a  means  of  adjusting  for  thicknc  ss 
of  glass  cover.  Frey  distinguishes  two  attributes  of  object-glasses,  viz., 
defining  power,  and  penetrating  or  resolving  power — penetrating  power 
and  resolving  power  being  considered  by  him  to  be  the  same  thing. 
Carpenter  enumerates  four  distinct  attributes  of  object-glasses,  viz., 
"  (1)  Defining  power,  or  the  power  of  giving  a  clear  and  distinct  image  of 
all  well  marked  features  of  an  object,  especially  of  its  boundaries;  (2)  itt» 
penetrating  power  or  focal  depth,  by  which  the  observer  is  enabled  to  look 
Into  the  structure  of  objects;  (3)  its  resolving  poicer,  by  which  it  enables 
closely  approximated  markings  to  be  distinguished;  and  (4)  the  flatness 
of  the  fietd  which  it  gives."  We  cannot  regard  any  of  these  classifica- 
tions, as  strictly  logical.  Beale  makes  no  formal  statement,  but  gives 
some  very  excellent  practical  directions  in  regard  to  the  selection  of  ob- 
jectives. 


66  SELECTION  AND  USE 

aberration,  or  both.  It  might  be  caused  by  a  want  of  finish  on 
the  surfaces  of  the  lenses,  but  this  is  seldom  the  case  in  prac- 
tice, except  where  the  objective  has  been  exposed  to  some  cor- 
roding fumes  or  liquids.  Old  objectives  that  have  been  very 
excellent  in  their  day,  sometimes  fail  in  defining  power,  from 
the  fact  that  the  surface  becomes  covered  with  a  greasy  deposit, 
very  slight,  it  is  true,  but  just  enough  to  destroy  the  efficiency 
of  the  glass.  Objectives  in  this  condition  should  be  returned 
to  the  makers  to  be  cleaned.  In  one  case  we  found  that  in  a 
lens  which  failed  to  show  anything  clearly,  the  •  difficulty  arose 
from  the  fact  that  the  cement  used  for  uniting  the  glasses  of 
the  combination  had  become  affected.  The  objective  was  by  a 
well-known  maker,  but  was  over  twenty  years  old 

Achromatism. — When  an  objective  shows  much  color,  it 
fails  to  define  well  except  by  monochromatic  light,  such  as  that 
obtained  by  passing  sunlight  through  a  cell  filled  with  the  blue 
solution  of  copper  in  ammonia.  A  very  slight  degree  of  color 
is  not  regarded  as  objectionable,  and  indeed  it  has  been  found 
almost  impossible  to  secure  the  requisite  angular  aperture  and 
absence  of  spherical  aberration  without  leaving  a  little  color. 
Some  of  the  best  objectives,  therefore,  show  such  objects  as 
the  P.  angulatum  with  decided  colors,  and  yet  well  resolved. 

Aberration  of  Form.— An  objective  may  appear  to  de- 
fine an  object  perfectly,  and  yet  give  a  very  distorted  figure  oi 
it,  just  as  a  cylindrical  mirror  gives  a  perfectly  definite,  though 
very  distorted,  image  of  objects  seen  reflected  in  it.  Aberra- 
tion of  form  may  arise  either  from  over  or  under  correction  oi 
the  spherical  aberration,  or  from  want  of  hoinogeniety  in  the 
glass  used  for  making  the  lenses,  or  from  a  want  of  perfection 
in  the  workmanship — the  surfaces  of  the  lenses  not  being  per- 
fectly spherical.  Sometimes  this  defect  is  shown  very  clearly 
on  one  side  of  an  objective,  while  the  other  side  is  not  affected, 
'and  this  fact  may  give  rise  to  very  curious  results  when  the 
objective  is  tried  on  different  stands,  and  with  oblique  light. 
Owing  to  a  variation  in  the  point  at  which  the  screw  threada 
begin  in  the  different  stands,  the  objective,  when  fairly  screwed 
up,  may  have  a  different  position  in  each,  as  regards  the  direction 


Otf  THE  MICKOSCOPS.  67 

from  which  the  illumination  comes.  The  consequence  is,  that 
an  objective  which  may  give  excellent  results  on  one  stand, 
may  foil  on  another.  Aa  easy  way  of  testing  this  fact,  is  by 
means  of  a  rotating  adapter.  Of  course  the  best  test  for  aber- 
rution  of  form  is  the  artificial  star,  though  in  the  hands  of  the 
beginner,  a  micrometer,  ruled  into  squares,  is  probably  t-'.e 
most  available  test.  Any  trace  of  the  defect  under  considei- 
ation  will  be  shown  by  the  lines  being  carved. 

When  the  lines  appear  curved,  from  the  fact  that  the  spherical 
aberration  has  not  been  properly  corrected,  the  nature  of  the 
error  may  be  determined  as  follows:  When  the  micrometer 
lines  are  widest  apart  at  the  centre  (like  the  lines  on  a  map  of  a 
hemisphere)  the  spherical  aberration  has  been  over-corrected. 
It  is  under-corrected  when  the  reverse  is  the  case. 

Aberration  of  form  is  one  of  the  worst  faults  with  which  a 
lens  can  be  affected,  and  experience  has  shown  us  that  it  is  the 
one  which  is  least  apt  to  be  detected  by  a  beginner.  An  objec- 
tive may  give  a  "beautiful"  image,  and  yet  be  worthless 
because  affected  with  this  defect. 

Flatness  of  Field.— If,  when  we  examine  a  perfectly  flat 
object,  every  part  included  in  the  field  of  view  is  clearly  in 
focus,  the  objective  is  said  to  have  a  flat  field.  Want  of  flat- 
ness of  field  is  shown  by  some  parts  of  a  fiat  object  being 
clear  and  well-defined,  while  other  parts  are  out  of  focus.  In 
general  it  happens  t'jat  where  this  defect  exists,  the  centre  aud 
circumference  of  the  objective  do  not  act  together. 

Angular  Aperture. — This  subject  has  given  rise  to  some 
of  the  most  vexatious  questions  connected  with  microscopy,  for 
a  discussion  of  which  we  must  refer  our  readers  to  the  pages  of 
the  microscopical  journals  published  during  the  past  few  years. 
The  views  which  have  been  promulgated  by  the  two  schools  into 
which  microscopists  have  been  divided  on  the  questions  affect- 
ing angular  aperture,  have  been  of  an  extremely  opposite 
nature,  and  few  scientific  disputes  have  been  waged  with  more 
bitterness  and  personalities  than  that  which  has  been  called  the 
"Battle  of  the  High  and  Low  Angles."  Now  that  the  smoke 
has  in  a  measure  cleared  away,  and  that  we  are  able  to  take  a 


68  SELECTION  AtfD  USE 

calm  view  of  the  actual  results  which  have  been  attained,  there 
is  little  room  for  doubting  that  the  wide  angles  have  gained  the 
victory  in  all  the  most  important  points.  In  a  subsequent 
section  we  shall  give  our  view  in  regard  to  the  circumstances 


Fig.  14.  Fig.  15. 

DR.  GORING'S  FIGURES  ILLUSTRATING  ANGULAR  APERTURE. 

which  ought  to  influence  the  beginner  in  the  selection  of  ob- 
jectives of  high  or  low  angles  ;  at  present  we  shall  confine  our- 
selves to  a  discussion  of  what  angular  aperture  is,  and  its 
influence  upon  the  working  qualities  of  the  objective. 

The  reader  who  will   carefully  l*ok  over  the  pages  of  the 
microscopic  journals  published  during  the  last  few  years,  will 


OF  THE  MICROSCOPE. 


probably  find  that  amongst  the  combatants  even  the  definition 
of  angular  aperture  seems  to  be  undecided,  all  of  which  is,  of 
course,  very  puzzling  to  a  beginner.  We  do  not  propose  to 
decide  this  vexed  question,  but  the  following  statement  will, 
we  hope,  enable  our  readers  to  form  a  clear  idea  of  what  the 
older  writers  meant  by  this  term. 

It  was  Dr.  Goring,  we  believe,  that  first  pointed  out  the 
special  advantages  of  high  angles,  and  suggested  the  use  of 
test  objects,  and  the  figures  on  the  preceding  page  were  used  by 
him.  to  define  and  explain  what  he  meant  by  angular  aperture. 

In  these  figures,  LL  and  I/I/  are  two  lenses  of  the  same 
magnifying  power,  but  different  angular  apertures.  It  will  be 
seen  that  the  cone  of  rays  proceeding  from  O,  is  substantially 
the  same  as  that  from  O',  but  that  the  lens  L  L  takes  in  a  larger 
part  of  the  cone  from  O,  than  the  lens  I/  I/  does  of  the  cone 
from  O'.  The  angles  L  O  L  and  L'  O'  I/  are  the  respective 
measures  of  the  angular  apertures  of  the  two  lenses. 

The  definition  of  angular  aperture, 
given  by  Goring,  has  been  followed 
by  all  subsequent  writers,  the  ac- 
companying figure  being  that  used 
by  Dr.  Carpenter  for  the  purpose  of 
explaining  and  defining  the  same 
thing.  In  this  figure  the  compound 
lenses,  1,  2  and  3,  are  parts  of  an 
achromatic  objective  without  its 
brass  mounting.  The  line,  a  c,  is 
the  front  of  the  objective,  and  b  is 
the  object.  In  this  case  the  angle. 
a  b  c,  is  the  angle  of  aperture,  and  it 
is  evident  that  if  a  c  had  been  made 
of  a  larger  diameter,  or  if  the  curva- 
tures of  the  lenses,  1,  2  and  3,  had  been  such  that  the  distance 
of  a  c  from  b  had  been  lessened,  the  angle,  a  b  c,  would  ha%  e 
been  increased.  The  points  that  we  have  mentioned  as  govern- 
ing the  angular  aperture  are  in  general  controlled  by  the 
character  of  the  glass  used,  and  by  the  formula  (as  regards 
curves,  distance,  etc.)  employed  by  the  optician,  and  lie  cannot 
deviate  from  them  without  an  entire  change  of  design. 


Fig.  16. 


70  SELECTION   A$D    USE 

Dr.  Goring1  devised  several  practical  methods  of  measuring 
the  angular  aperture  of  different  objectives,  and  he  gives  a  very 
full  and  clear  description  of  an  arrangement  adapted  to  his  own 
instrument,  in  which  the  foot  was  made  to  rotate  on  a  carefully 
centered  and  graduated  base,  and  modifications  of  this  plan 
have  since  been  adopted  by  several  of  our  prominent  micro- 
scope makers.  It  has  been  pointed  out,  however,  that,  as 
usually  employed,  the  angle  which  is  measured  by  this  method 
is  not  the  angle  a  b  c,  and  although  the  angle  which  is  actually 
determined  may  be  of  great  value  in  determining  the  qualities 
of  an  objective,  it  can  hardly  be  denied  that  to  apply  the  term 
angular  aperture  to  any  other  angle  than  the  one  that  has  been 
described,  must  tend  to  introdiice  a  confusion  of  terms,  and  is 
altogether  wrong.  But  in  order  to  be  strictly  accurate,  -this 
statement  presupposes  a  condition  which  is  unfortunately  too 
frequently  overlooked,  and  that  is  that  all  the  proportions  and 
curves  of  the  different  parts  of  the  objective  are  perfect.  To 
make  this  clear,  let  us  assume  that  Fig.  16  truly  represents  an 
objective  of  which  the  diameters,  curves,  and  distances  apart  of 
the  several  lenses,  are  absolutely  correct  for  the  kind  of  glass 
used.  It  is  obvious  that  by  slightly  increasing  the  diameter  of 
lens  1,  or  by  changing  the  relations  of  the  three  lenses,  the  ap- 
parent angular  aperture  might  be  considerably  increased.  We 
would  soon  find,  however,  that  this  increase  was  of  no  advan- 
tage, because  after  passing  a  certain  degree  the  back  combina- 
tions would  fail  to  utilize  the  additional  rays  admitted,  and  the 
result  would  be  that  the  performance  of  the  lens  would  be  in- 
jured rather  than  improved.  Therefore,  although  in  &  properly 
constructed  lens  the  angle  a  b  c  shows  the  true  angular  aperture, 
this  will  not  be  the  case  if  the  optician  has  made  the  front,-  a  c, 
larger  than  it  ought  to  be  in  relation  to  the  other  parts  of  the 
objective.* 

It  is  an  unfortunate,  but  an  indisputable  fact,  that  this  very 
mistake  is  frequently  committed  by  makers  of  objectives,  who 
either  have  not  studied,  or  do  not  appreciate  the  qualities  of 
lenses  of  wide  angles.  They  therefore  make  objectives  which 

*  The  reader  -who  desires  to  pursue  this  subject  will  find  it,  fully  dis- 
cussed in  Dr.  Blackhani's  paper  on  Angular  Aperture,  which  has  been 
published  in  a  separate  form, 


OF   THE   MICROSCOPE.  71 

admit  widely  divergent  pencils,  but  which  fail  to  give  distinct 
and  sharply-defined  images.  Such  lenses,  when  measured  by 
the  older  methods,  indicate  extraordinarily  large  angles,  and 
yet  fail  to  have  a  resolving  power  at  all  commensurate  with 
the  aperture  claimed.  Indeed,  when  such  objectives  are 
diaphragmed  down  to  moderate  angles,  their  performance  is 
frequently  greatly  improved.  The  action  in  this  case  is  similar 
to  that  of  the  diaphragms  in  the  hand  magnifiers,  shown  in 
Figures  2  and  3.  In  these  simple  lenses,  it  is  found  that  by 
cutting  off  the  outer  rays,  which  do  not  come  to  the  same  focus 
as  the  central  ones,  the  definition  is  greatly  improved. 

It  is  a  fact  no  longer  open  to  dispute,  that  the  efficiency  of 
an  objective,  supposing  it  to  be  properly  corrected,  may  be 
more  nearly  expressed  by  its  angular  aperture  than  by  its  focal 
length.  For  example,  if  we  have  two  objectives,  one  of  one- 
fifteenth  of  an  inch  focal  length,  and  150°  angular  aperture,  and 
another  of  one-fifth  of  an  inch  focal  length,  and  180°  angular 
aperture,  the  latter,  although  magnifying  only  one-third  as 
much  as  the  other,  will  actually  show  more  of  the  more  delicate 
struct tiral  features  of  any  object ;  and  if  its  magnifying  power 
be  increased  by  suitable  eye-pieces  until  it  is  equal  to  the  mag- 
nifying power  of  the  one-fifteenth, -it  will  give  a  better  image 
than  the  latter.  Our  own  experience  has  long  since  convinced 
us  of  the  truth  of  this,  and  we  believe  that  it  is  the  opinion 
held  to-day  by  all  who  are  familiar  with  glasses  of  both  kinds. 
Prof.  Abbe,  who  is,  perhaps,  the  highest  living  authority  on  the 
mathematics  of  the  microscope,  goes  so  far  as  to  say  that  the 
angular  aperture  of  an  objective  is  an  exact  expression  of  its 
power,  and  he  gives  u  table  in  which  he  lays  down  the  number  of 
lines  to  the  inch  which  may  be  resolved  by  any  given  aperture. 

It  is  not  difficult  to  give  a  practical  demonstration  of  the  im- 
portance of  a  comparatively  wide  angle  of  aperture,  for  as  we 
have  just  said,  object-glasses  possessing  this  feature  are  capable 
of  giving  important  results,  which  cannot  be  obtained  by  means 
of  lenses  of  lower  angle.  Thus,  when  we  examine,  by  means 
of  a  superior  French  triplet  of  one-sixth  of  an  inch  focus, 
the  silicious  remains  of  certain  very  minute  plants  of  the  species 
Plaurosigma  Balticum,  we  are  able  to  see  certain  lines  or  mark- 
ings which  exist  .upon  their  surfaces.  That  we  may  be  able 


72  SELECTION   AND    USE 

to  see  these  lines,  it  is  necessary  that  the 
stand  be  a  good  one,  and  that  Ihe  light 
be  very  carefully  managed,  but,  even 
with  the  most  perfect  arrangements  or- 
dinarily used,  wo  cannot,  with  such  an 
objective,  discover  similar  markings  upon 
the  Pleurosigma  Angulatum,  although 
they  exist  there  in  great  perfection.  But 
if  for  tbe  French 'sixth  we  substitute  a 
first-class  objective  of  less  than  half  its 
magnifying  power,  but  of  wider  angular 
aperture,  we  shall  be  able  to  see  the  lines 
quite  distinctly.  We  have  now  before  us 
an  objective  of  four- tenths  of  an  inch 
focus,  which  does  not  correct  for  thick- 
ness of  cover,  but  whicb,  with  any  ordi- 
nary thickness  of  covering  glass,  is  capa- 
ble of  resolving  the  lines  on  the  Angu- 
latum perfectly,  and  we  have  seen  objec- 
tives of  even  lower  magnifying  power 
which  Avould  accomplish  the  same  thing. 
That  the  effect  depends  here  chiefly 
upon  angular  aperture,  was  shown  very 
clearly  by  Dr.  Goring,  from  whose  work 
we  take  the  following  figures,  engraved 
from  S2ven  drawings  showing  the  ap- 
pearances presented  by  the  scale  of  a 
butterfly's  wing,  viewed  with  the  same 
magnifying  powers,  but  different  angular 
apertures.  A  well  corrected  lens  of  wide 
aperture  showed  the  scale  as  in  G;  re- 
ducing the  aperture,  while  all  else  re- 
mained the  same,  the  appearance  was  as 
shown  in  F,  and  by  successive  reduc- 
tions the  stages  shown  in  E,  D,  C,  B, 
and  A,  were  reached.  The  slightest  ex- 
amination shows  that  features  which  were 
quite  distinct  under  a  high  angle,  became 
invisible  when  the  angle  was  reduced.  Fig.  17. 


OF  THE  MICROSCOPE.  73 

This  quality  of  objectives  of  large  angles,  whereby  they 
are  capable  of  showing  distinctly  delicate  lines  or  dots  placed 
very  closely  together,  is  known  as  resolving  power.  In  the  early 
days  of  microscopy,  it  was  called  penetrating  power,  the  term 
penetrating  having  been  applied  to  that  quality  of  the  telescope 
by  which  it  is  enabled  to  show  separately  the  individual  stars 
of  which  the  nebulae  are  composed.  In  the  telescope  this  was 
supposed  to  depend  upon  space-penetrating  power  as  distin- 
guished from  mere  magnifying  power,  and  this  space-penetrat- 
ing power  was  found  to  depend  very  largely  upon  angular  aper- 
ture. In  the  case  of  the  microscope,  however,  it  is  now  gene- 
rally agreed  that  what  was  called  penetrating  power  in  the 
telescope,  shall  be  called  resolving  power,  while  to  the  term 
penetrating  power  an  entirely  different  meaning  has  been  given. 

Mere  resolving  power,  however,  or  the  power  of  showing 
separately  lines  placed  very  closely  together,  is  not  the  only 
valuable  feature  of  well-corrected  object-glasses  of  high  angles. 
They  show  delicate  Hues  and  fibres,  and  enable  us  to  make  out 
differences  of  structure  which  are  entirely  invisible  to  lenses 
of  low  angles.  Thus,  for  example,  it  has  been  found  during 
recent  researches,  that  the  delicate  flagella  of  certain  monads 
can  be  seen  perfectly  with  high  anglo  lenses,  while  with  very 
excellent  glasses  of  low  angular  aperture  they  are  quite  invis- 
ible. The  same  fact,  probably,  holds  true  in  regard  to  the  ulti- 
mate fibres  of  nerves  and  similar  objects. 

The  researches  of  Lister  and  Ross  formed,  as  we  have  just 
stated,  the  first  great  step  in  the  direction  of  better  correction  and 
increased  angular  aperture.  "Whereas,  40°  to  65°  had  previously 
been  regarded  as  very  high  angles,  even  in  objectives  of  the 
shortest  focal  distance,  Ross,  in  his  objectives,  soon  attained 
an  aperture  of  132°  to  135°  and,  working  with  the  glass  at  that 
time  available,  this  was  pronounced  the  highest  attainable 
angle.  Attempts  had  previously  been  made  to  obtain  a  higher 
angle  by  the  use  of  the  glass  which  Faraday  devised  for  optical 
purposes,  and  which  is  in  fact  a  borate  of  lead.  But  this  com- 
pound is  so  easily  tarnished  and  disintegrated,  that  it  was 
found  impracticable  to  use  it.  It  happened,  however,  that 
a  young  American  backwoodsman,  Charles  A.  Spencer,  of 
Canastota,  N.  Y.,  a  graduate  of  Hamilton  College,  had  his 


74  SELECTION   AND    USE 

attention  called  to  this  subject,  and  after  careful  study  lie  con- 
cluded that  if  he  could  only  procure  a  durable  glass  of  greater 
refracting  power  than  that  ordinarily  attainable,  the  angular 
aperture  might  be  greatly  increased.  He  at  once  went  to  work, 
and  after  many  experiments  he  succeeded  in  producing  a  glass 
•which  enabled  him  to  attain  immediately  an  angular  aperture 
of  146°.  As  early  as  1857  he  had  produced  a  l-12th  with  an 
angular  aperture  of  178°.  His  objectives  had  corresponding  ex- 
cellence in  other  directions,  and  from  that  time  forward  this 
country  has  been  noted  for  the  excellence  of  its  objectives,  and 
especially  for  their  great  resolving  power.  "We  may  note,  in 
passing,  that  glass  of  great  refractive  power,  combined  with 
sufficient  hardness  and  durability,  is  now  produced  as  a  regular 
article  of  commerce. 

Penetrating  Power.— As  previously  stated,  penetrating- 
power,  in  the  early  days  of  microscopy,  meant  precisely  what  is 
now  understood  by  resolving  power.  Now,  however,  penetra- 
ting power  is  usually  understood  to  mean  the  extent  to  which 
an  object-glass  shows  the  depth  or  thickness  of  an  object.  It 
is  obvious  that  such  a  result  can  only  be  produced  by  the  lens 
showing  several  layers  of  images,  all  of  which  are  equally  in 
focus,  and  consequently  equally  visible  at  the  same  time.  In 
other  words,  instead  of  bringing  only  one  given  point  sharply 
to  a  focus,  the  lens,  -which  has  great  penetrating  power,  will 
bring  several  other  points  lying  above  and  below  this  point, 
equally  to  a  focus.  Hence,  as  Dr.  Pelletan  well  observes  in  his 
work,  "Le  Microscope"  :  "  From  a  purely  theoretical  point 
of  view,  an  objective  with  penetration  is  in  reality  a  defective 
objective." 

As  this  subject  is  one  of  great  importance,  we  have  intro- 
duced the  two  engravings,  Figures  18  and  19,  for  the  purpose 
of  making  it  clear  to  our  readers.  In  Figure  18  we  have  shown 
a  small  lens  of  half-inch  focus,  and  with  a  very  narrow  angle. 
Figure  19  shows  a  lens  of  the  same  focal  length,  but  with  an 
angle  of  90°.  The  points  at  which  the  lines  a  c  and  b  c  in  Fig. 
18,  and  a'  c'  and  V  c'  in  Fig.  19  cross  each  other,  are  the  respec- 
tive foci  of  the  two  lenses  at  the  centre  of  the  field  of  view. 
Across  the  points  where  the  lines  a  c  and  b  c,  and  a'  c'  and  b'  d 


OF   THE   MICROSCOPE.  75 

cross,  we  have  drawn  three  lines,  which  may  be  supposed  to 
represent  respectively  three  layers  of  cells  in  a  section  mounted 
for  microscopical  examination.  If  the  reader  will  examine  the 
relation  of  these  lines  to  the  crossing  lines  in  Fig.  18,  he  will 
find  that  the  lines  a  c  and-  b  c  coincide  completely  between  the 
upper  and  under  lines,  and  even  outside  of  them,  forming,  in 
fact,  a  single  line.  In  Figure  19,  on  the  other  hand,  it  is  only 
the  middle  line  that  passes  through  the  point  at  which  the  lines 
coincide  ;  at  the  point  where  the  upper  and  lower  lines  pass, 


Fig.  18.  Fig.  19. 

the  lines,  a'  c'  and  V  <f,  have  already  separated  to  such  an  extent 
as  to  form  distinct  lines.  If  we  regard  the  lines,  a  b,  b  c  and 
a'  b',.  b'  tf,  as  rays  of  light,  it  is  evident  tha^.  while  those  in  Fig. 

18  will  form  images  of  the  upper  and  lower  lines,  those  in  Fig. 

19  cannot  do  so.* 

Until  within  a  few  years  it  has  been  accepted,  as  a  thoroughly 
demonstrated  fact,  that  penetrating  and  resolving  power  always, 
of  necessity,  exist  in  inverse  ratio  to  each  other,  for  it  is  always 
found  that,  other  things  being  equal,  resolving  power  increases 
with  the  angle  of  aperture,  while  penetrating  power  decreases. 
Of  late,  however,  it  has  been  claimed  that  certain  lenses 
of  great  resolving  power  possess  considerable  penetrating 
power. 

*  Those  who  wish  to  examine  tliis  subject  more  closely,  would  do  well 
to  examine  an  article  on  "Penetration  in  Objectives,"  by  Dr.  Geo.  E. 
Blackliam,  published  in  the  American  Journal  of  Microscopy,  for  July, 
1880,  and  the  discussions  which  follow  iu  succeeding  numbers. 


76 


SELECTION   AXD   USE 


In  attempting  to  reach  a  sound  conclusion  on  this  point,  it 
must  be  borne  in  mind  that  resolving  power  does  not  depend 
wholly  upon  angular  aperture.  Two  objectives  of  the  same 
angle  may  have  very  different  powers  of  resolution,  on  account 
of  the  degree  of  perfection  to  which  the  corrections  have  been 
carried,  and  it  is  quite  possible  that  a  lens  of  great  resolving 
power  may  have  a  lower  angle  than  another  objective  which 
it  excels  in  this  respect.  In  this  case  the  lens  of  greatest  resolv- 
ing power  might  also  have  the  greatest  penetration.  Wo  confess, 
however,  that  we  cannot  see  how  great  penetration  can  be  com- 
bined with  very  high  angular  aperture,  and  in  this  view  we  be- 
lieve that  we  are  in  accord  with  the  majority  of  our  best  micro- 
scopists.  Want  ol  penetration  has  been  urged  against  objec- 
tives of  high  angular  aperture  as  a  serious  defect  in  many  in- 
vestigations in  histology  and  natural  history.  The  advocates 
of  high  angles  claim  that  the  best  way  to  avoid  this  difficulty 
is  to  use  objectives  of  less  magnifying  power,  and  to  combine 
them  with  high  eye-pieces.  It  is  claimed  that  in  this  way  the 
same  amplification  and  penetration  may  be  obtained  as  with 
low-angled  objectives,  and  with  greatly  increased  distinctness 
and  improved  definition. 

Where  stereoscopic  effect  is  required  by  means  of  the  ordin- 
ary binocular,  low-angled  objectives  give  better  results  than 
those  of  higher  angle — one  reason  for  this  being  that  the- 
corrections  of  such  objectives  are  not  so  easily  disturbed  as 
those  of  the  others.  But  it  is  a  curious  fact  that  low-power 
objectives  of  high  angle,  when  used  monocularly  for  examining 
opaque  objects,  frequently  give  an  impression  of  stereoscopic 
effect  which  is  startling.  We  feel  certain  that  no  one  that  has 
his  attention  once  called  to  this  point  can  ever  forget  it. 

The  late  Henry  J.  Clark,  in  an  article  contributed  by  him  to 
Silliman's  Journal,  many  years  ago,  suggested  another  meaning 
for  the  term  "penetration"  in  objectives.  He  referred  to  the 
power  which  objectives  of  high  angle  and  perfect  correction 
have  of  pasing  through  and  ignoring,  as  it  were,  the  various 
layers  of  cells  or  other  elements  which  lie  above  the  special 
layer  in  focus,  which  latter,  when  seen  by  a  high-angle  lens,  is 
clear  and  distinct,  while  all  the  others  are  invisible.  Mr.  Stod- 
der,  who  has  recalled  my  attention  to  this  article,  which  I  had 


OF  THE   MICROSCOPE.  77 

read  many  years  ago  but  had  forgotten,  suggests  that  this  be 
called  the  penetration  of  high  angles  in  contradistinction  to  the 
penetration  of  low  angles  as  understood  by  Dr.  Carpenter, 
Prey,  and  others.  Where  the  latter  kind  of  penetration  obtains, 
all  the  layers  are  visible  at  once,  as  we  have  previously 
explained. 

Working  Distance.— Considered  as  a  quality  of  an  ob- 
jective, working  distance  is  the  distance  between  the  front  lens 
and  the  object,  and  it  is  this  distance,  and  this  only,  which  can 
guide  us  in  forming  an  estimate  of  the  quality  of  any  given  ob- 
jective, as  regards  this  particular  feature.  In  practice,  this 
distance  is  diminished  by  the  amount  of  metal  used  in  setting, 
which  projects  in  front  of  the  lenses,  and  also  by  the  thickness 
of  the  cover  glass.  It  is  evident,  however,  that  these  are  mere 
accidental  circumstances  which  can  be  readily  changed,  and 
which  have  nothing  whatever  to  do  with  the  quality  of  the  ob- 
jective as  regards  its  optical  characteristics.  Some  writers 
have  defined  working  distance  to  be  the  space  between,  the 
front  of  the  lens  and  the  upper  surface  of  the  cover  glass  ;  but 
it  is  very  evident  that  if  we  accept  this  definition  we  can  no 
longer  regard  "  working  distance  "  as  a  quality  of  the  objective, 
since  an  objective  which  might  be  notable  for  its  great  working 
distance  with  ordinary  cover  glasses,  would  have  no  working 
distance  at  all  if  very  thick  covers  were  used. 

To  avoid  this  difficulty,  the  term  frontal  distance  has  been 
introduced,  and  is  used  by  some  writers  to  signify  the  quality 
heretofore  recognized  as  working  distance.  But  since  the  term 
"working  distance "  has  obtained  a  firm  foothold  in  micro- 
scopical literature,  as  expressing  an  important  quality  or  feature 
of  objectives,  we  cannot  see  how  any  other  meaning  can  be  at- 
tached to  it  than  that  given  above. 

Working  distance  is  a  very  important  feature  in  all  lenses, 
and  good  working  distance  is  specially  valuable  to  beginners. 
There  are  many  objectives  in  market  that  have  to  be  brought 
so  close  to  the  object  that  ordinary  covering  glass  cannot  be 
used,  and  even  with  the  thinnest  glass,  the  distance  between 
the  objective  and  the  object  is  such  that  great  skill  and  care  are 
required  to  avoid  accidents.  Such  objectives  do  excellent  work 


78  SELECTION  AND   USE 

in  the  hands  of  experienced  microscopists,  but  beginners  will 
find  it  difficult  to  use  them.  Objectives  of  very  high  angular 
aperture  have  in  general  very  short  working  distances,  but  there 
are  great  differences  in  this  respect  amongst  the  products  of 
different  makers.  Working  distance  does  not  depend  upon 
angular  aperture  alone. 

Immersion  and  "  Homogeneous "  Lenses.— Ob- 
jectives which  require  a  drop  of  liquid  between  the  front  lens 
and  the  covering-glass  of  the  object,  are  now  in  common  use, 
and  have  been  deservedly  received  with  general  favor.  The 
liquid  employed  serves  two  important  purposes.  In  the  first 
place,  it  partially  extinguishes  two  of  the  glass  surfaces  (the 
front  surface  of  the  objective  and  the  upper  surface  of  the 
covering-glass),  and  thus  it  prevents,  to  a  considerable  extent, 
the  loss  of  light  which  always  occurs  at  these  surfaces  ;  and  in 


Fig,  20. 

the  second  place,  it  enables  the  objective  to  gather  up  rays 
which  otherwise  would  pass  outside  of  it.  In  the  ordinary  im- 
mersion objective,  water  is  the  liquid  which  is  used,  and  its 
action  will  be  readily  understood  from  an  inspection  of  the 
accompanying  diagram,  where  c  is  the  front  of  the  objective ; 
Y  is  the  cover-glass  ;  E  the  space  between  the  front  lens  and 


OF  THE  MICEOSCOPE.  79 

the  cover ;  o  the  object,  and  o  of  a  ray  of  light  proceeding 
from  that  object.  It  is  obvious  that  when  the  space,  E,  is 
filled  with  air,  the  ray,  o  of,  after  passing  through  the  cover- 
glass  and  suffering  refraction  within  it,  will,  on  emerging,  pass 
on  in  a  direction  parallel  to  its  original  path,  and  so  will  pursue 
the  line,  I  d,  and  pass  outside  of  the  objective.  But  if  the 
space,  E,  be  filled  with  water,  it  will  take  the  course  /  c,  and  so 
will  enter  the  front  lens. 

It  is  evident  that  if  a  fluid  of  greater  refracting  power  than 
water  were  used,  the  result  would  be  still  more  marked.  This 
consideration  led  to  the  use  of  glycerine  instead  of  water,  with 
notably  good  results,  and  finally  lenses  have  been  constructed 
in  which  a  liquid  of  the  same  refractive  and  dispersive  power, 
as  the  glass  of  the  thin  cover,  is  used,  and  such  lenses  are  known 
as  "objectives  of  homogeneous  immersion,"  or,  for  short, 
"homogeneous  objectives  " — the  word  homogeneous  meaning  of 
the  same  kind,  and  applying  to  the  liquid  and  the  cover-glass. 

An  immerson  fluid,  perfectly  homogeneous  with  the  cover- 
glass,  would  evidently  obviate  all  necessity  for  what  is  known 
as  "cover  correction, "'provided  certain  other  conditions  in  the 
mounting  of  the  object  and  the  construction  of  the  microscope 
always  remained  the  same,  and  most  of  the  objectives  made  by 
Zeiss  on  the  homogeneous  system  have  no  such  correction. 
But  since'  a  difference  in  the  quality  or  thickness  of  the  me- 
dium in  which  the  object  is  mounted,  or  a  difference  in  the 
length  of  the  tube  or  body  of  the  microscope  used  with 
the  objective,  gives  rise  to  serious  disturbances,  American 
makers  have  preferred  to  make  their  best  homogeneous  ob- 
jectives "adjustable,"  as  it  is  called  ;  that  is,  provided  with  the 
usual  means  for  cover  correction. 

It  is  a  general  impression  that  immersion  objectives  are  more 
difficult  to  use  than  dry  ones  ;  such  is  not  the  case,  however. 
Immersion  objectives,  with  the  same  degree  of  effort  and  skill, 
give  finer  results  than  dry  objectives  of  the  same  power,  though 
of  course  for  the  ordinary  rapid  work  of  searching  for  known 
objects,  dry  lenses  are  to  be  preferred  as  being  less  trouble- 
some, for  it  is  undeniable  that  it  is  some  trouble  to  apply,  the 
liquid  and  clean  the  lens,  and  also  the  cover,  and  where  a  large 
number  of  observations  are  to  be  made,  even  trifling  delays 


80  SELECTION  AND  USE 

must  be  considered.  Some  makers  have  endeavored  to  avoid 
this  difficulty  by  supplying  double  fronts  (a  wet  and  a  dry)  to 
their  objectives,  while  others  have  constructed  objectives  which 
work  either  wet  or  dry,  according  as  the  arrangement  used  for 
the  ordinary  cover-adjustment  is  set  to  the  one  or  the  other. 

Lens  Systems— Duplex  Fronts.— Formerly  the  term 
"system"  was  applied  only  to  the  entire  combination  forming 
the  objective,  and  we  had  "immersion  systems,"  "correction 
systems,"  etc.  At  present  the  word  is  used  also  to  denote  the 
individual  combinations  of  two  or  more  pieces  of  glass,  which, 
when  arranged  together,  form  the  whole  objective,  as  will  be 
understood  from  Fig.  16,  where  1,  2  and  3  form  the  separate 
systems,  each  composed  of  two  pieces  of  glass.  Such  a  com- 
bination (the  figure  of  which  is,  of  course,  only  diagramatic)  is 
said  to  form  a  three-system  lens.  Very  low  powers,  formed  of 
two  achromatic  lenses,  are  said  to  be  two-system  ;  four  com- 
binations, four-system,  etc. 

Some  years  ago,  Mr.  Tolles  brought  out  objectives  of  remark- 
able power,  which  were  known  as  "duplex  fronts,"  or  "four- 
system  objectives.  These  objectives  were  so  called  because  a 
second  lens  (nearly  hemispherical)  was  added  to  the  front  com- 
bination, and  this  additional  lens,  added  to  the  three  already 
in  use,  made  a  four-system  arrangement. 

French  Triplets.— A  few  years  ago  these  objectives  were 
used  quite  extensively.  They  are  so  called  because  they  ori- 
ginated in  the  country  after  which  they  are  named,  while  to 
further  distinguish  between  them  and  objectives  constructed 
according  to  the  principles  laid  down  by  Lister,  the  latter  were 
known  as  the  English  form.  Good  makers  of  the  English  form 
are  now  found  in  the  United  States,  France,  Germany,  Austria, 
and  Italy  ;  and  the  French  pattern  is  made  in  many  of  the 
cities  of  Europe  outside  of  France,  although  as  yet  neither  the 
English  nor  the  American  opticians  have  been  able  to  manu- 
facture them  at  prices  which  can  compete  with  those  of 
continental  Europe.  The  best  of  the  so-called  French  ob- 
jectives consist  simply  of  lenses  in  which  the  chromatic  aber- 
ration is  corrected  by  the  usual  plan  of  making  each  lens 


OP   THE  MICROSCOPE.  81 

of  two  different  kinds  of  glass,  while  the  spherical  aberration 
is  corrected  partly  by  the  form  of  the  lens,  but  chiefly  by  re- 
ducing the  aperture,  and  by  properly  combining  a  series  of 
single  lenses,  which,  however,  are  never  especially  adjusted  to 
each  other,  as  in  the  English  forms.  Each  objective,  in  its 
most  perfect  condition,  consists  of  three  lenses  screwed  to- 
gether, and  in  the  lower  powers  these  lenses  may  be  separated 
and  used  either  singly  or  in  combinations  of  two  or  three.  As 
the  magnifying  power  obtained  with  two  lenses  is  less  than 
that  obtained  by  three,  the  defects  of  the  double  combination 
are  not  as  obvious  as  they  would  be  if  the  magnifying  power 
were  equal  to  that  of  the  triple  combination.  As,  however,  the 
spherical  aberration  in  the  case  of  a  single  lens,  whether  it  be  a 
plain  lens  or  an  achromatic  combination,  is  always  greater  than 
that  of  a  doublet,  and  the  aberration  in  the  doublet  greater 
than  that  in  the  triplet,  it  is  never  a  good  plan  to  attempt  to 
obtain  a  low  or  moderate  power  by  separating  the  lenses  of  a 
high  power  objective,  and  using  them  singly  or  in  twos.  Any 
person  having  a  few  French  objectives  at  hand  who  will  try  this 
and  attempt  to  secure  the  same  magnifying  power  by  the  use  of 
two  lenses,  and  also  by  the  use  of  three,  the  latter  being  a  regu- 
larly adjusted  combination,  will  find  that  the  results  obtained 
by  the  use  of  the  latter  are  far  superior  to  those  afforded  by  the 
former.* 

Considering  their  quality,  these  French  objectives  are  re- 
markably cheap.  Thus  a  French  No.  2,  which  is  nearly  equiv- 
alent to  the  one-fourth  objective  of  the  English  and  American 
opticians,  can  be  bought  for  $5,  while  the  cheapest  student's 
objective  of  this  power  would  cost  at  least  double  that  sum.  In 
addition  to  this,  the  French  objective  may  be  divided  so  as  to 
afford  two  other  objectives  of  about  three-quarters  and  one 
inch  eacl),  and  although  the  performance  of  these  is  far  inferior 
to  English  or  American  objectives  of  the  same  power,  they  are 

*In  making  such  a  trial,  it  is,  of  course,  necessary  to  use  lenses  of  equal 
quality  in  both  cases,  since  the  quality  of  the  professedly  achromatic 
French  objectives  in  market  varies  very  much.  We  have  seen  objectives 
of  this  class  of  the  same  magnifying  power,  one  of  which  would  not  resolve 
ihe  markings  on  the  scales  of  the  clothes-moth's  wing,  while  the  other 
would  resolve  the  Pleurosigma  BaUicum. 


82  ELECTION  AtfD    tJSE 

capable  of  showing  a  great  deal  that  is  interesting  and  instruc- 
tive. Ten  or  twelve  years  ago  these  lenses  were  the  only  ones 
furnished  with  microscopes  costing  less  than  $50,  and  in  the 
very  cheap  instruments  the  different  powers  were  always 
obtained  by  the  division  of  one  doublet  or  triplet,  which 
was  thus  made  to  yield  two  or  three  different  objectives. 
Those,  however,  who  cannot  afford  American  objectives,  and  who 
wish  to  do  work  that  is  of  some  real  value,  are  advised  never  to 
separate  their  objectives,  or  at  least  never  to  separate  any  but 
the  very  lowest — that  is  the  No.  1,  and  against  even  this 
we  would  protest  were  it  not  for  the  fact  that  cheap  lenses 
of  lower  power  than  the  half-inch  are  seldom  found  in  market; 
and  therefore,  no  other  course  except  the  division  of  a  No.  1 
is  left  to  us  when  we  wish  to  use  a  lens  of  lower  power.  But 
this  system  of  dividing  is  of  ten.  carried  too  far,  and  we  find  mi- 
croscopes in  market  which  are  furnished  with  No.  2  or  No. 
3  objectives  which  are  divided  when  lower  powers  are  needed. 
This  is  decidedly  wrong.  If  a  power  lower  than  No.  1  be  needed, 
it  may  be  admissible  to  divide  this  number,  because  this  is  in 
general  the  only  course  left  to  us,  but  a  No.  2  should  never 
be  divided  for  the  purpose  of  obtaining  an  objective  equivalent 
to  a  No.  1. 

The  value  of  the  numbers  assigned  to  the  different  French  ob- 
jectives varies  according  to  the  fancy  of  the  maker,  but  those 
of  the  better  class  usually  found  in  market  are  about  as  follows : 

Number 1       2        3       4        5         6 

Corresponding  focus  in  parts  of  an  iuch 1-2    1-4    1-G    1-8    1-10    i-12 

Frey,  in  his  recent  work  on  the  microsc*  p  3,  regards  the 
English  system,  whereby  the  focus  is  expressed  in  inches,  as 
"peculiar."  It  certainly  is  "peculiarly  "  definite  and  positive, 
instead  of  being  indefinite  and  arbitrary,  as  is  the  system 
adopted  by  the  French  and  German  opticians.  According  to 
the  English  and  American  systems,  an  objective  of  an  inch 
focus  ought  to  be  the  same,  no  matter  by  what  maker  it  has 
been  constructed,  but  when  designated  after  the  plan  which 
Frey  seems  to  prefer,  it  is  impossible  to  tell  what  the  focus  of 
the  lens  may  be,  and  consequently  what  its  power  is.  Thus— 
a  No.  2  of  Nachet  has  a  focus  of  half  an  inch,  while  a  No. 
2  of  Hartnack  has  a  focus  of  one  inch,  and  a  No.  2  of  the 


OF    THE   MICROSCOPE.  £3 

ordinary  French  objectives  is  about  a  quarter  of  an  inch  in 
focal  length.  As  it  is  often  useful  to  the  microscopist  to  know 
the  powers  of  the  different  objectives  made  by  prominent  con- 
tinental makers,  we  give  the  focal  lengths  of  the  objectives  of 
Nachet,  Hartnack,  and  Gundlach,  premising,  however,  that 
by  so  doing  we  by  no  means  intend  to  class  these  objectives 
with  ordinary  French  triplets. 

Nachet's  ordinary  objectives  are  as  follows: 

Number 0  12345 

Focus  in  inches 2  1          1-2-      1-4         1-5        1-8 

The  immersion  and  correction  objectives  of  the  same  maker 
are  as  follows: 

Number 6  7  8  9  10  11  12 

Focus  in  inches 1-10      1-14      1-15       1-26        1-30        1-40        1-50 

Hartnack's  objectives  of  recent  construction  are  as  follows: 

Number 123456789 

Focus  in  inches....  2         1        3-4      1-2      1-4      1-5       1-6      1-9     1-11 

Hartnack's  new  objectives  with  immersion  and  correction  are 
as  follows: 


No.  Focus  in  inches.* 

9 1-12 

10 , 1-16 

11 1-18 

12 1-21 

13 ..  1-25 


No.  Focus  in  inches. 

14 1-28 

15 1-33 

16 1-40 

17. 1-45 

18..  .  1-50 


The  following  is  Gundlach's  scale: 

No.  Focus  in  inches.  (  No.  Focus  In  inches. 

I....    , 1  Via    .  ..  1-12 


II 1-2 

III 1-3 

IV 1-4 

V .1-8 


Vlb 1-12 

Vila 1-16 

VII6 1-16 

Yin    1-24 

IX    .  .  1-32 


Via  and  Vila  are  not  adjustable  for  thickness  of  cover, 
while  VI6  and  VII£  are.  Vila,  VII6,  VIII  and  IX  are  immer- 
sion lenses. 


84  SELECTION  AND    USE 

Testing  Objectives.— At  first  sight  it  would  seem  to  be 
the  easiest  thing  in  the  world  to  test  an  objective,  and  find  out 
whether  or  not  it  is  capable  of  doing  certain  work,  but  a  little 
experience  soon  teaches  those  who  are  not  too  self-conceited, 
that  it  is  the  easiest  thing  in  the  world  to  be  deceived.  We 
have  seen  those  who  considered  themselves  the  most  capable 
of  judges,  condemn  lenses  that  had  received  the  approbation  of 
the  ablest  microscopists  in  the  world — lenses  too  that  had  shown 
their  efficiency  by  doing  really  good  work;  showing  that  even 
those  who  consider  themselves  very  expert,  may  sometimes  arrive 
at  wrong  conclusions.  If  this  is  the  case,  then,  with  men  of 
training  and  experience,  how  can  a  beginner,  who  has  had  no 
experience,  hope  to  be  able  to  form  a  correct  judgment  in  re- 
gard to  the  quality  of  an  objective  ? 

But  while  it  is  difficult,  or  perhaps  impossible,  to  pronounce 
a  positive  opinion  in  regard  to  the  quality  of  an  objective, 
especially  those  made  for  some  of  the  higher  departments  of 
microscopic  work,  it  is  in  general  easy  for  those  who  have  had 
experience,  to  form  a  judgment  in  regard  to  ordinary  objectives, 
or  at  least  those  designed  for  ordinary  purposes.  The  ability 
to  form  such  a  judgment  depends  rather  upon  experience  and 
a  comparison  with  the  work  of  other  glasses  than  upon  a  refer- 
ence to  any  special  standard;  and  therefore,  as  a  general  rule, 
we  would  advise  beginners  who  are  about  to  purchase  objectives, 
to  obtain  the  advice  and  assistance  of  some  skilful  friend. 
To  those  who  cannot  obtain  such  assistance,  we  offer  the  follow- 
ing hints. 

The  great  difficulty  in  the  way  of  arriving  at  a  decision  in 
regard  to  the  quality  of  an  objective,  is  the  want  of  a  standard 
with  which  to  judge  its  performance.  When  we  examine  the 
image  which  an  objective  gives  of  any  object,  it  is  very  difficult 
to  decide  whether  or  not  that  image  truly  represents  the  ob- 
ject. Take,  for  example,  the  podura  scale:  wide  differences  of 
opinion  exist  as  to  its  structure,  and  how  it  ought  to  look;  sup- 
pose, then,  that  two  objectives  show  entirely  different  appear- 
ances of  this  object,  who  shall  decide  which  one  is  correct  ? 
And  if,  even  in  the  case  of  expert  microscopists,  this  holds 
true  absolutely,  which  it  does,  how  shall-  a  beginner  determine 
that  the  images  which  he  sees  through  an  objective  are  true  or 


OP  THE  MICKOSCOPE.  85 

false  ?  In  some  departments,  the  most  earnest  and  long-con- 
tinued discussions  have  been  maintained  in  regard  to  the  ac- 
curacy or  inaccuracy  of  certain  images  as  seen  by  professional 
microscopists,  and,  strange  to  say,  these  disputes  affect  the 
very  tests  most  commonly  used,  viz. ,  the  Podura  scale  and  the 
test  diatoms. 

Makers  of  objectives,  and  skilful  microscopists,  being  aware 
of  the  fallacies  which  beset  examinations  of  this  kind,  resort  to 
certain  artificial  standards  of  which  the  construction  is  posi- 
tively known,  and  which  should  therefore  give  appearances 
conforming  to  this  known  strueture.  Numerous  tests  of 
this  kind  have  been  suggested,  but  the  only  ones  generally 
accepted  are  the  artificial  star*  and  ruled  glass  plates.  Of  the 
latter,  ordinary  micrometers  answer  a  very  good  purpose,  but 
the  most  delicate  tests  are  the  famous  ruled  plates  of  M.  Nobert. 

In  the  examination  of  objectives,  there  are  a  few  simple  gene- 
ral rules  which  must  be  observed  by  the  microscopist  if  he 
\vould  secure  accurate  results. 

The  first  important  point,  and  one  to  which  sufficient  attention 
is  not  generally  given,  is  the  health  of  the  observer  at  the  time  of 
making  the  trial.  The  eye  is  a  very  delicate  organ,  and  the 
slightest  derangement  of  the  stomach  or  nerves  affects  it  to  an 
extent  that  few  persons  realize.  We  have  an  obj  jet-glass  of 
comparatively  low  power,  with  which,  when  in  good  personal . 
health,  we  find  no  difficulty  in  resolving  the  P.  angulatum, 
though  a  very  slight  disturbance  of  the  digestive  organs,  ren- 
ders t'le  lines  perfectly  invisible. 

It  must  also  be  remembered  that  in  the  case  of  such  delicate 
observations,  personal  peculiarities,  irrespective  of  health  o:- 
sickness,  exert  a  marked  influence,  so  that  it  does  not  follow 
that  what  one  observer  sees,  all  can  see.  We  have  frequently 


"The  artificial  star  is  a  very  minute  globule  of  mercury,  obtained  by 
crushing  a  small  drop  by  means  of  a  smart  tap  with  a  flat  slip  of  iron  or 
ivory.  This  globule  is  made  to  act  as  a  small  convex  mirror,  reflecting 
the  light  of  a  lamp,  candle  or  window.  It  is  not  mentioned  bymoden 
writers  on  the  microscope  (Carpenter,  Hogg,  Beale,  Frey,  etc.),  but  is 
used  by  some  of  our  best  opticians.  Dr.  Roystou  Piggott,  has  recently 
revived  its  use.  Goring  devoted  considerable  space  to  an  account  of  the 
oest  methods  of  using  it. 


86  SELECTION  AND    USE 

seen  those  who  could  not  distinguish  lines  that  were  visible  to 
others,  and  we  have  also  met  those  to  whom  an  objective,  in 
which  the  chromatic  errors  wero  very  obvious,  seemed  to  be 
perfect.  This  probably  arose  from  a  kind  of  color  blind- 
ness. We  have  also  met  eyes  which  distorted  objects,  and 
those  which  saw  fringes  of  color  round  objects  viewed  through 
an  objective  of  generally  recognized  excellence. 

Attempts  have  been  made  to  get  rid  of  the  errors  arising 
from  personal  peculiarities  (or  what  may  perhaps  be  called  the 
"personal  equation")  by  employing  photography,  it  being 
assumed  that  if  a  lens  will  give  an  image  which  can  be  photo- 
graphed, it  must  give  an  image  that  may  be  seen,  and  that 
whatever  is  photographed  must  of  necessity  be  a  real  im  ige. 
But  from  the  known  fact  that  the  foci  of  the  chemical  and 
visual  rays  do  not  coincide,  and  that  the  corrections  required 
in  the  one  case  are  not  those  calculated  to  give  the  best  results 
in  the  other,  we  have  little  faith  in  photography  as  the  best 
test  of  the  excellence  of  an  objective,  except,  of  course,  in  those 
cases  where  photographic  work  is  the  chief  purpose  in  view. 
Lenses  intended  to  transmit  an  image  to  the  eye  must  be  tested 
by  the  eye,  and  if  certain  eyes  show  peculiarities  not  possessed  by 
the  average  eye,  then  lenses  must  be  corrected  specially  for  them. 

It  is  scarcely  necessary  to  say  that  when  an  objective  is  put 
upon  its  trial,  the  stand  and  means  of  illumination  ought  to  bo 
such  as  will  do  it  justice.  The  best  stand  in  the  world  cannot 
make  a  good  objective  out  of  a  poor  one,  but  a  poor  stand  will 
give  poor  results  even  with  the  best  objective.  The  eye-pieces 
also  should  be  of  good  quality,  and  if  an  objective,  which  the 
microscopist  has  reason  to  believe  is  a  good  one,  fails,  it  should 
not  be  condemned  until  it  has  been  tried  with  eye-pieces  either 
by  the  same  maker,  or  of  a  known  standard  of  excellence.  And 
we  must  also  remember  that  it  is  not  sufficient  to  examine  an 
objective  in  combination  with  a  shallow  eye-piece,  or  one  giving 
a  low  magnifying  power.  An  objective  may  perform  very  well 
if  used  with  low  eye-pieces,  and  utterly  fail  when  a  higher 
power  is  applied.  M  )st  makers  of  objectives  test  their  glasses 
under  eye-pieces  of  very  high  power — a  quarter  and  even  an 
eighth  of  an  inch  focus,  or  what  would  be  equivalent  to  H  or  K 
on  the  usual  scale. 


OF  THE  MICBOSCOPE.  87 

The  room  in  which  the  test  is  made  must  also  be  a  subject  of 
careful  selection.  Very  many  of  our  best  microscopes  are  used 
in  our  large  cities;  at  least  they  are  very  generally  examined 
there  with  a  view  to  purchase.  Now,  those  who  are  familiar 
with  the  subject,  know  that  during  the  day  time  the  buildings 
along  the  principal  thoroughfares  in  our  large  cities  are  in  such 
a  state  of  constant  vibration,  that  geod  results  are  rendered  im- 
possible, and  therefore  that  an  objective  and  stand  which, 
under  such  circumstances,  fail  to  resolve  difficult  tests,  or  to 
define  clearly,  should  not  on  that  account  be  condemned. 

.The  illumination  employed  must  also  receive  careful  atten- 
tion. An  objective  which  readily  resolves  the  P.  angulatum  by 
central  illumination,  when  lamp  light  or  good  daylight  is  used, 
may  fail  when  poor  daylight  is  employed.  Special  directions 
on  this  point  are  given  under  the  head  of  light  and  illumiii- 
tion,  and  therefore  we  would  merely  say  here  that  an  objec._ .  j 
which  has  been  tested  only  by  the  dull  blue  light  of  a  northern 
sky,  cannot  be  said  to  be  inferior  because  it  has  failed  either  in 
resolving  or  defining  power. 

On  the  other  hand,  we  must  not  place  too  high  an  estimate 
on  an  objective  which,  by  the  aid  of  monochromatic  light,  (the 
blue-cell,  for  example,)  has  resolved  certain  difficult  tests.  It 
is  not  uncommon  to  find  that  lenses  of  a  quarter-inch  focus 
will,  with  blue  light,  resolve  the  AmpJiipleura  pettucida,  but 
fail  completely  with  ordinary  light.  Even  eighths  and  tenths 
by  the  same  makers,  and  of  a  grade  quite  as  good  as  the  fourths 
just  mentioned,  fail  to  resolve  the  Amphipleura  by  ordinary 
illumination,  even  when  well  managed.  The  aid  which  is 
derived  from  blue  light  in  the  resolution  of  difficult  diatoms  is 
unquestionable,  but  it  is  not  quite  so  clear  that  this  kind  of 
light  gives  the  same  assistance  in  the  matter  of  definition.  O'.ir 
own  experience  leads  us  to  believe  that  the  real  assistance  de- 
rived in  the  latter  case  is  very  slight.  Therefore,  we  do  not 
regard  it  as  a  very  high  recommendation  for  ordinary  work  that 
a  lens  can  resolve  the  AmpJiipleura  by  blue  light.  We  have, 
however,  seen  a  fourth  which  would  resolve  the  Amphipleura  by 
the  light  of  an  ordinary  hand  lamp,  aided  by  Wenham's  reflex 
illuminator.  The  objective  was  made  by  Tolles,  and  manipu- 
lated by  him. 


88  SELECTION  AKD    USE 

To  determine  the  quality  of  an  objective  it  is  best  to  take  up 
in  succession  the  several  features  which  we  have  just  detailed, 
and  examine  its  efficiency  in  each  of  these  directions.  First  of 
all,  the  denning  power  should  be  carefully  tried,  this  being  the 
most  important  quality  that  a  glass  can  possess.  No  special 
test  can  be  named  for  this,  and  in  fact  the  formation  of  a  cor- 
rect judgment  in  regard  -to  it  will  depend  more  upon  the 
experience  of  the  observer  than  upon  any  particular  rules  that 
can  be  laid  down.  As  Carpenter  well  says:  "An  experienced 
microscopist  will  judge  of  the  denning  power  of  a  lens  by  the 
quality  of  the  image  which  it  gives  of  almost  any  object  with 
which  he  may  be  familiar."  To  which  we  may  add  that  the 
inexperienced  microscopist  will  in  general  fail  to  detect  a  want 
of  denning  power,  no  matter  what  object  he  may  examine. 

Tha  chief  points  seem  to  be  that  the  outlines  should  be  sharp 
and  clear,  the  blacks  black,  and  the  other  natural  colors  clear 
and  distinct.  Frey  compares  the  image  given  by  a  good  lens 
to  a  good  copper  plate,  or  a  print  with  sharp  letters,  and  no 
illustration  could  be  more  to  the  point.  He  also  states  that  an 
objective  which  is  deficient  in  this  respect,  is  best  tested  with 
u  pretty  strong  eye-piece.  In  our  own  experience  we  have 
found  no  surer  test  of  excellence  than  this  ;  an  objective  which 
is  deficient  in  denning  power,  is  sure  to  "break  down  "  under* 
a  high  eye-piece.  Deep  eye-piecing  does  not  affect  the  resolv- 
ing power  of  a  lens  to  the  samo  extent  that  the  defining  power 
is  lessened,  and  therefore,  the  fact  that  a  glass  shows  Hues 
under  a  high  eye-piece,  is  not  an  absolute  demonstration  of  its 
excellence  as  regards  definition.  At  the  same  time,  it  will  be 
found  that  considerable  angular  aperture  is  absolutely  neces- 
sary to  enable  any  glass  to  bear  deep  eye-piecing,  because  with- 
out this,  the  loss  of  light  is  so  great  that  nothing  can  be  seen 
clearly.  Hence  the  truth  of  the  somewhat  paradoxical  state- 
ment, that  an  objective  may  be  really  good  under  a  low  eye- 
piece, and  yet  fail  under  a  high  one. 

With  the  English  opticians  a  favorite  test  for  definition  is 
the  Podura  scale.  Unfortunately,  however,  the  structure  of 
this  scale  and  even  the  identification  of  the  scale  itself,  seem 
to  be  a  matter  of  doubt.  Page  after  page  has  been  written  for 
the  purpose  of  showing  how  the  Podura,  scale  ought  to  lopk,  and 


OF  THE   MICKOSCOPE.  89 

still  the  question  seems  to  b3  undecided.  Carpenter,  in  his  last 
edition  (page  702)  says:  "The  sharp  and  distinct  bringing-ont 
of  the  '  exclamation  marks  'of  the  Podura  scale,  constitutes, when 
it  coexists  with  the  greatest  practicable  freedom  irom  color,  and 
with  adequate  'focal  depth  '  or  'penetrating  power,' the  most 
valuable  proof  of  the  fitness  of  an  Objective  of  high  power  for 
the  purpose  of  scientific  work. " 

To  give  our  readers  an  idea  of 

how  the  podura  scale  ought  to  look, 
we  give  a  figure  copied  from  the 
CD  graving  published  by  the  late 
Richard  Beck,  in  his  work  on  the 
microscope.  The  figure  shows 
"  the  appearance  of  the  Podura 
scale  when  the  adjustment  of  the 
object  glass  is  correct  and  the  mark- 
ings are  in  focus."  The  objective 
used  was  a  one-eighth,  giving  a 
Fig'  21.  magnifying  power  of  1,300  diame- 

ters. 

It  is,  we  believe,  generally  conceded  that  in  the  present  state 
of  the  art,  perfect,  correction  for  color  cannot  be  obtained,  but 
so  long  as  the  residuary  chromatic  abberration  does  not  inter- 
fere with  the  defining  power  of  the  objective,  it  cannot  be  re- 
garded as  an  objection.  And  yet  we  have  seen  a  would-be  critic 
reject  a  very  excellent  lens  because  it  showed  a  little  color, 
while  he  was  loud  in  his  praise  of  another  lens  which,  although 
more  perfect  in  this  respect  was  almost  worthless  otherwise. 
Like  specks  of  dirt  on  an  eye-piece,  which  do  no  harm  and  are 
never  even  seen  by  experienced  microscopists,  slight  color  and 
want  of  flatness  of  field  are  the  betes  noirs  of  beginners.  They 
are  the  defects  which  are  most  easily  detected,  and  the  detec- 
tion gives  the  critic  an  air  of  knowledge  which  is  to  him  a 
source  of  great  pride. 

The  best  English  and  American  opticians  now  slightly  under- 
correct  their  best  objectives,  so  that  the  field  shows  a  slightly 
greenish  hue,  while  any  prominent  markings  on  the  object, 
such  as  the  dots  on  the  Angulatum  stand  out  clear  and  well  de- 
fined and  of  a  very  delicate  ruby  tint.  According  to  Frey  the 


90  SELECTION  AND    USE 

majority  of  continental  makers  adopt  the  opposite  plan. 
Their  lenses  are  over- corrected,  and  objects  show  a  bluish 
border. 

A  want  of  correction  for  color  is  shown  when  thin  objects  with 
many  cross  lines  are  examined,  especially  with  slightly  oblique 
light.  As  a  test  for  achromatism  in  low  powers,  Carpenter 
prefers  a  section  of  coniferous  wood,  showing  the  glandular 
dots.  He  also  recommends  the  tracheaB  of  insects,  but  almost 
any  lined  object  will  answer  the  purpose. 

The  existence  of  aberration  of  form  is  best  proved  by  the  use 
<)f  a  fine  micrometer  or  a  Nobert's  plate.  When  this  defect  is 
very  marked,  it  is- easily  seen  in  the  curved  and  distorted  lines 
of  which  the  image  consists,  but  such  a  state  of  things  exists 
only  in  extreme  cases.  Where  this  distortion  is  not  very  glar- 
ing, it  may  be  necessary  to  compare  the  magnified  image  of  the 
lines  in  the  stage  micrometer  with  straight  lines  ruled  on  a  thin 
plate  of  glass  laid  on  the  diaphragm  of  the  eye-piece—in  other 
words  with  an  eye-piece  micrometer  in  which  the  ruled  lines  are 
quite  long. 

For  testing  for  flatness  of  field  and  aberration  of  form,  Frey 
recommends  ' '  a  slide  thickly  smeared  with  India  ink,  in  which 
small  circles  or  other  figures  are  scratched  with  the  point  of  a 
fine  needle.  *  *  *  If  the  instrument  is  adjusted  with  trans- 
mitted light  for  such  a  circle,  it  should  appear  sharply 
cut  on  the  black  ground,  and  not  surrounded  by  a  halo 
of  light.  If  the  circle  is  then  brought  out  of  focus,  it 
gradually  enlarges,  while  its  sharp  borders  disappear,  with- 
out spreading  n  strong  halo  of  light  either  inwards  or  outwards 
over  the  black  field. " 

The  angular  aperture  of  an  objective  can  be  determined 
accurately  only  by  measurement,  and  this  is  something  that 
beginners  will  hardly  attempt.  To  measure  accurately  the 
angular  aperture  of  an  objective,  is  a  task  requiring  con- 
siderable skill  and  knowledge,  and  most  of  the  appli- 
ances furnished  by  microscope-makers  for  this  purpose,  fail 
to  give  accurate  results.  It  must  be  remembered  that  in 
measuring  the  angle  of  an  objective,  we  must  comply  with 
the  same  rules  that  govern  the  accurate  measurement  of 
any  other  angle.  Dr.  Carpenter,  in  his  work,  gives  a  method 


OF  THE  MICROSCOPE.  91 

which  he  calls  "the  simplest  and  most  convenient."  We  ven- 
ture to  say,  that  none  but  an  expert  can  obtain  by  it  results 
that  are  anywhere  near  accurate,  especially  with  high  powers. 
We  therefore  consider  that  any  directions  upon  this  subject, 
addressed  to  beginners,  would  be  worse  than  useless. 

Since,  however,  the  resolving  power  of  an  objective  depends 
in  a  large  measure  upon  its  angular  aperture,  we  may  feel 
pretty  certain  that  an  objective  which  fails  to  resolve  tests 
suited  to  its  magnifying  power,  is  deficient  in  angular  aperture, 
unless,  indeed,  its  inefficiency  should  arise  from  want  of  defining 
power,  which  may  be  tested  by  other  means.  Of  ordinary  work- 
ing lenses,  the  half  inch  ought  easily  to  resolve  the  Pleurosigma 
Balticum\  the  quarter  inch  should  resolve  the  P.  angulatam  by 
oblique  light,  and  those  of  a  fifth  or  sixth  inch  focus  should 
resolve  the  latter  test  by  axial  or  central  light.  An  eighth, 
tenth,  or  twelfth,  ought  to  resolve  all  the  diatoms  on  the  Probe 
Platte  below  the  17th.  It  is  true  that  objectives  of  a  quarter 
inch  focus  have  been  made  to  resolve  everything  on  the  Probe 
Platte,  but  such  glasses  cost  too  much  to  render  it  likely  that 
they  will  fall  into  the  hands  of  ordinary  students.  Twelfths 
and  sixteenths  should  go  through  the  Probe  Platte  easily.  If 
they  cannot  do  this  it  would  be  better  to  take  a  lower  power  of 
better  quali  by,  and  use  it  with  a  higher  eye-piece. 

We  must  also  be  on  our  guard  against  an  old  source  of  error — 
the  use  of  lined  tests  which  vary  from  the  accepted  standard.* 
Great  differences  exist  in  the  different  specimens  of  the  var- 
ious test  objects  that  are  used,  some,  owing  to  individual 
characteristics  and  the  methods  employed  in  mounting,  being 
much  more  easily  resolved  or  shown  than  others.  Conse- 

*" The  proof  objects  [finely  lined  insect  scales]  originally  discovered  by 
me,  are  sufficient  for  that  purpose  in  honest  hands,  and  when  nsed  with  (he 
precautions  I  have  pointed  out.  But  it  is  well  known  that  they  have  been 
shamefully  abused,  owing  to  the  various  facilities  of  resolution  which 
exist  between  different  specimens  of  lined  objects,  the  external  charac- 
ters of  which  closely  resemble  each  other;  so  that  it  may  be  said  that 
there  are  proof  objects  to  suit  the  capacities  of  all  microscopes;  nay.  they 
arc  actually  perverted  to  the  purpose  of  deceiving  the  unscientific  part  of 
the  public  in  a  much  more  effectual  manner,  than  could  possibly  have 
been  done  without  them." — Goring.  What  is  true  of  the  scales  used  by 
Goring  is  also  true,  though  perhaps  not  to  the  same  extent,  of  diatoms. 


92  SELECTION    AND    USE 

quently,  because  an  objective  resolves  ono  specimen  of  the  P. 
Angulalum,  it  does  not  follow  that  it  will  resolve  all  others.  One 
of  the  most  important  steps  in  the  direction  of  uniformity  in 
this  respect,  at  least  so  far  as  testing  the  resolving  power  of 
objectives  is  concerned,  is  furnished  by  the  test-plate  (Probe 
Platte)  of  J.  D.  Moller.  Upon  a  slide  of  the  usual  size,  he  ar- 
ranges twenty  diatoms,  carefully  selected  as  to  cleanness,  and 
also  as  to  resolvability.  Those  that  he  has  chosen  for  the  pur- 
pose are  named  in  the  accompanying  table.  They  are  arranged 
on  the  slide  in  a  line  which  is  about  a  quarter  of  an  inch  in 
length,  the  beginning  and  end  of  the  row  being  marked  by  a 
specimen  of  Eupodiscus  Argus,  Ehrbg.  The  table  on  the  oppo- 
site page  gives  the  closeness  of  the  lines  and  the  direction  of 
the  markings  in  these  diatoms  according  to  the  best  authorities. 
In  this  connection  it  must  not  be  forgotten,  however,  that  mere 
closeness  is  not  the  only  feature  which  makes  a  series  of  lines 
easy  or  difficult  of  resolution.  Every  micrometer  maker  knows 
that  of  two  sets  of  lines,  both  ruled  at  10,000  to  the  inch,  one 
may  be  much  more  difficult  to  resolve  than  the  other.  The 
strength  of  the  individual  lines  has  as  much  to  do  with  it  as 
the  mere  distance  at  which  they  are  placed  apart.  Holler's 
Probe  Platte  is  furnished  of  two  kinds,  dry  and  in  balsam,  the 
latter  being,  of  course,  by  far  the  most  difficult  test.  It  is  an 
unfortunate  fact,  however,  that  even  with  all  the  care  and  skill 
exercised,  even  the  test-plates  of  Herr  Moller  do  not  always 
conform  to  a  standard;  and,  therefore,  were  it  not  for  the  facts 
just  stated,  it  would  seem  that  the  most  trustworthy  tests  are 
the  ruled  plates  of  M.  Nobert. 

It  is  not  difficult  to  test  an  objective  of  moderate  power  for 
flatness  of  field,  provided  we  have  on  hand  a  suitable  object. 
For  this  purpose  a  thin  section  of  wood,  or  of  an  echinus  spine, 
is  generally  chosen.  For  low  powers  a  very  excellent  test  is 
one  of  those  micro-photographs  which  are  so  common.  One 
showing  a  sentence  or  sentences  should  be  chosen  in  prefer- 
ence to  a  picture,  since,  unless  the  field  of  view  be  flat,  the  whole 
of  the  letters  will  not  be  clearly  readable  at  once,  while  in  a 
picture  the  effect  known  as  aerial  perspective  mny  give  rise  to  an 
impression  of  want  of  flatness  of  field.  In  applying  tests  for 
flatness  of  field,  it  is  of  course  obvious  that  we  must  make  sure 


OF  THE  MICROSCOPE. 


l_l  _     —     —     —    V- &    —  —     — 

50        GO  7-1  p  ot  jj>-  co       P  J-1 


O    O    00   ~3    Ci    Cn    rf*- 


M 

*Q  £ 
5s  oT  y 

III 

5:1 3- 

b  r1 

Elf 


izjoophrfj^oQ     QS^gsQCQ^iz! 
,f°*d  cj  ?r^S— ,3  o'^-'S'o'n  P  s"^ 

2.  3  3-  S  55  t=.  ||  %  gs  N   s   tf  §  a  B   g. 


33 


r   (/-  ^ 

roS  ^ 


g«  P   B 


11  d'5- s. o  »». 

Jfi^fg 

liiiEff 

1SIB 


a  2 


3-  02 

-»3!£<*  3 
g0^? 


b 
Sm 


v3  H 

Is 
II 


_f  I 

y  i 

it! 

ST.  O    ?° 

IrSal 
S2.o   E 

g  §   g 

^11 

f    02 

H  3 
cr  • 
M  • 


g§  §  S 

lill 

p'35'aT  | 


fBI 


p 


=.=  ^3 

2  3  S  3 

?lll 

oft  ii «   § 

3  3^"o2 
:    w"  3 

.     05 


III 

;H 

iTe 

o'^3    2- 

§/3T 

S"   02   '• 

3  3  : 


HI 


CO    tO    h- 

tz!  ^  1-3 

5   5'  3- 

C3     ^ 

o'  0    ^ 
&S   ~ 

*5l 

3    &I4 

sf  i 

Is-31 
V  IP 

£<*£ 

?     •     05 


?r^ 

5  05  s 


H  «  H 


H  H  H  H  H  H 

2     8»     ED     p     p     2 

a   a    s    s    s   s 


re    o    o    ^ 

i  i  i  i 

o    o    o    ® 


£g: 


?! 
i 


-a  en       rf* 

S"  o        o         o 

«O    -3          tO  tf>-    Cn    CO    00 


^•CTi^csciospco       S 

CnCOl-*O»OCOCOO!i*0 

v  2  c  a 

o"        o'ooctoog.p 

rf^-  CO    C5    *J    OO    L-O    CO    "     i-*» 


8: 


sB^I 


94  SELECTION   AND    USE 

that  the  test  used  is  itself  flat.  Common  glass  slides  are  not 
flat,  and  as  they  are  used  for  the  cheaper  kinds  of  micro- 
photographs,  this  fact  may  give  rise  to  errors  if  we  are  not  care- 
ful in  our  selections.  The  best  slides  are  cat  from  glass  plate 
which  has  been  ground  and  polished  so  that  the  sides  are  per- 
fect planes,  and  it  is  this  kind  only  that  should  be  used.  Care 
should  also  be  taken  to  see  that  the  object  lies  flat  on  the  slide, 
and  is  not  distorted  by  the  cover.  We  have  seen  an  objective 
condemned  because  it  did  not  show  all  the  diatoms  in  the  field 
of  view  in  focus  at  once,  when  the  fact  was  that  the  diatoms 
were  attached  to  t'le  cover  which  was  slightly  wavy  as  cov.  rs 
often  are.  When  it  is  suspected  that  the  fault  is  not  in  the  objec- 
tive, but  in  the  slide  or  cover,  the  object  should  be  carefully 
passed  across  the  field  of  view,  and  the  changes  in  focus  noticed. 
This  will  in  general  tell  where  the  defect  lies,  for  if  the  part 
that  is  apparently  foggy  should  move  as  the  object  nlbves,  it 
shows  that  the  object  itself  is  not  flat.  It  has  been  'recom- 
mended by  high  authority  to  test  objectives  for  fl  itness  of  field 
by  strewing  some  fine  powder  on  a  slide  and  seeing  whether  all 
the  grains  are  in  focus  at  once.  For  obvious  reasons  this  is  a 
very  unreliable  method. 

Penetration  in  low  powers  is  perhapa  most  readily  deter- 
mined by  the  examination  of  opaque  objects  of  considerable 
thickness.  The  round  pollen  grains  of  the  hollyhock,  and  the 
rounded  forms  of  the  polycystina  are  excellent  tests  for  objec- 
tives of  an  inch  or  inch  and  a  half.  Lower  powers  ought  t > 
show  coarser  objects  in  all  their  dimensions,  while  for  those  of 
medium  power  the  coarser  cellular  tissue  of  plants  answers  very 
well.  It  is  more  difficult  to  indicate  a  good  test  for  penetra- 
tion in  the  higher  pjvvers,  in  which,  by  the  way,  wo  have  often 
seen  want  of  penetration  mistaken  for  want  of  flatness  of  field. 
This  arose  simply  from  the  fact  that  scarcely  any  object  is 
absolutely  flat,  and  hence,  as  explained  under  another  head, 
the  curvature  of  the  object  is  sometimes  taken  as  an  indica- 
tion of  a  defect  in  the  objective.  Want  of  good  working 
distance  makes  itself  obvious  during  the  examination  of  any 
object  suited  to  the  objective. 


OF  THE  MICROSCOPE.  95 


OX   THE  SELECTION  OF  A  MICROSCOPE  FOR  PRAC- 
TICAL PURPOSES. 

Tlie  object  of  all  the  information  given  in  the  preceding 
pages,  is  to  enable  the  re.ider  not  only  to  understand  the  con- 
struction of  the  microscope,  but  to  select  one  judiciously  ;  and, 
therefore,  in  every  section  we  have  offered  hints  bearing  in  this 
direction.  We  now  propose  to  give  the  reader  such  special  in- 
structions in  regard  to  the  choice  of  an  instrument  as  are 
necessary  in  addition  to  those  previously  offered. 

Those  who  will  examine  the  catalogues  of  the  various  makers 
of  microscopes,  will  find  the  instruments  divided  into  first, 
second,  third,  fourth,  etc.,  classes,  the  microscopes  in  each 
succeeding  class  in  this  descending  scale  being  smaller  and  less 
complete  than  those  in  the  one  preceding  it.  The  first-class 
microscopes  of  almost  all  our  prominent  makers  are  large, 
beautifully-made  instruments.  They  are  provided  with  me- 
chanical movements  in  every  part,  whereby  the  utmost  delicacy 
and  precision  in  making  an  adjustment,  and  in  recording  it, 
may  be  obtained,  and  as  a  natural  consequence,  such  instru- 
ments are  quite  expensive.  Indeed,  it  will  often  be  found  that 
the  hanging  and  adjustment  of  the  mirrors  alone,  of  these  fine 
stands,  cost  as  much  as  an  entire  microscope  of  the  lower 
grades. 

Second-class  stands  are  usually  smaller  than  those  of  the  first 
class,  but  they  are  frequently  very  complete  as  regards  their 
adjustments  and  arrangements.  Microscopes  of  the  third  and 
fourth  class  are  usually  much  simpler  in  their  construction,  as 
well  as  less  in  size.  In  addition  to  these  different  classes,  most 
manufacturers  offer  models  adapted  to  special  purposes,  so  that 
in  setting  out  to  procure  a  microscope,  the  beginner  is  very  apt 
to  be  bewildered  unless  he  has  the  guidance  of  some  judicious 
friend.  Of  course  those  who  are  mere  dealers  always  urge  the 
purchase  of  the  most  complete  and  expensive  microscope  which 
the  funds  of  the  purchaser  will  enable  him  to  procure,  whereas 


96  SELECTION  AND  USE 

a  comparatively  cheap  microscope  would  frequently  answer  his 
purpose  far  better.  The  reader  will  of  course  bear  in  mind 
that  by  "  dealer  "  we  do  not  refer  to  experienced  opticians,  such 
as  are  most  of  our  microscope  makers.  From  these  men  the 
student  will  be  pretty  sure  to  get  sound  advice  and  efficient 
assistance.  But,  as  is  well  known,  every  dealer  in  spectacles 
sets  himself  up  as  an  optician,  and  professes  to  be  competent 
to  give  advice  in  regard  to  the  purchase  of  a  microscope  ;  and 
the  microscope  which  these  men  always  advise  the  purchaser  to 
procure  is  the  one  that  will  afford  them  the  largest  commissions.  It 
is  unnecessary  to  say  that  this  is  not  always  the  instrument 
that  will  afford  the  greatest  amount  of  satisfaction  to  the  be- 
ginner in  microscopy. 

In  selecting  a  microscope,  regard  must  be  had,  not  only  to 
the  excellence  of  the  instrument,  but  to  its  adaptability  to  the 
purpose  for  which  it  is  intended,  and  to  the  person  who  is  to 
use  it.  A  complicated  and  expensive  compound  microscope,  if 
placed  in  the  hands  of  a  person  having  little  experience  or  skill, 
would  evidently  be  worse  than  wasted,  while  to  attempt  to  con- 
duct elaborate  and  delicate  investigations  by  means  of  a  cheap 
non-achromatic  instrument,  would  simply  be  to  throw  away 
time,  and  wantonly  incur  the  risk  of  seriouc  errors.  And  yet  no 
mistake  is  more  frequently  made.  A  microscope  is  wanted  ;  the 
purchaser  is  liberal  with  his  means,  and  he  is  saddled  with  an 
expensive  instrument  entirely  unsuited  to  his  requirements. 
Or,  on  the  other  hand,  a  physician  or  student  of  limited  means 
requires  an  instrument,  and.  being  unable  to  afford  the  price  of 
a  really  good  one,  he  is  induced  to  purchase  a  cheap  affair, 
whose  indications,  when  applied  to  the  subjects  for  which  he 
requires  it,  are  entirely  unreliable  ;  whereas,  he  ought  to  be 
told  that  if  he  cannot  afford  a  microscope  which  is  at  least  pro- 
vided with  good  objectives,  and  the  necessary  facilities  for  using 
them,  he  ought  to  leave  microscopy  in  its  applications  to  medi- 
cine and  physiology  alone.  We  feel  it  the  more  necessary  to 
be  emphatic  on  this  point,  from  the  fact  that  cases  involving 
such  errors  have  so  often  come  under  our  own  observation. 
Thus  we  have  seen  cheap  French  instruments,  with  poor  triplets, 
in  the  hands  of  physicians,  and  used  in  cases  where  the  safety 
of  the  pa-tient  depended  upon  a  correct  diagnosis  ! 


OF  THE  MICROSCOPE.  97 

The  first  point  to  be  decided,  is  tlie  kind  of  microscope  that 
should  be  procured — that  is  to  say,  whether  a  simple  or  a  com- 
pound microscope  is  wanted.  If  the  student  is  desirous  of 
working  on  objects  under  the  microscope — making  dissections 
of  flowers,  etc. — a  simple  microscope,  of  any  of  the  kinds  de- 
scribed in  a  previous  chapter,  will  suit  his  purpose  best.  It 
should  be  arranged  on  a  stand,  and  this  he  can  either  do  him- 
self, or  he  can  procure  one  of  the  many  dissecting  microscopes 
which  he  will  find  described  in  the  catalogues  of  the  manufac- 
turers. For  such  purposes  a  simple  microscope  is  indispensa- 
ble, though  when  we  come  to  examine  the  results  of  our  work, 
(dissections,  etc.)  a  -good  compound  microscope  is  equally 
necessary. 

Attempts  have  been  made  to  combine  these  two  forms  of  the 
microscope  in  one  instrument,  and  with  considerable  success, 
so  far  as  increasing  our  power  to  examine  our  work  as  it  pro- 
gresses is  concerned.  This  is  accomplished  by  so  arranging 
the  simple  microscope  that  a  compound  body  with  its  eye- 
piece may  be  slipped  on  or  off,  as  required.  And  by  using  a 
single  concave  lens  as  an  eye-piece,  we  can  secure  considerable 
magnifying  power  and  great  working  distance,  while  at  the 
same  time  the  object  is  seen  erect  and  not  inverted,  as  when 
the  ordinary  compound  body  is  used.  This  form  is  called  the 
"Brueke  Loupe" — and  is  too  little  known  in  this  country. 
But  no  such  makeshift  can  take  the  place  of  a  good  compound 
microscope,  although  it  may  form  a  useful  and  convenient  ad- 
dition to  the  student's  outfit. 

The  selection  of  a  compound  microscope  will  frequently  be 
determined,  not  only  by  the  wants,  but  by  the  means  of  the 
purchaser.  To  those  who  are  obliged  to  put  up  with  a  cheap 
microscope,  the  only  advice  that  we  can  give  is  to  get  the  best 
they  can  for  the  money,  and  as  newer  and  better  microscopes 
are  constantly  coming  into  market,  the  best  thing  the  student 
can  do  is  to  procure  the  latest  catalogues  of  the  different 
dealers,  and  examine  and  compare  what  they  have  to  offer. 
There  is  no  microscope  in  market  to-day  that  will  not  probably 
be  excelled  by  something  better  in  less  than  twelve  months 
from  this  time. 

To  those  whose  means  enable  them  to  procure  whatever  they 


98  SELECTION  AND   USE 

may  require,  a  word  of  caution  is  also  necessary.  If  your 
studies  require  the  employment  of  the  very  best  instrumental 
appliances,  by  all  means  procure  either  a  first-class  stand  by 
some  well-known  maker,  or  a  microscope  made  to  suit  your 
special  requirements.  But  at  the  same  time  do  not  fail  to  pro- 
vide yourself  with  one  of  what  may  be  called  the  third-class  in- 
struments ;  that  is.  a  microscope  of  moderate  size,  and  destitute 
of  those  complicated  arrangements  which  are  the  glory  of  all 
first-class  stands.  With  such  a  microscope  you  will  be  able  to 
do  twice,  yes  thrice  as  much  work  as  with  the  large  heavy 
stands.  The  physician  or  working  naturalist  that  procures  a 
very  expensive  stand  for  every-day  use,  makes  a  great  mistake. 
It  is  very  well  to  have  such  a  stand  for  special  occasions,  but 
for  "  ordinary  work  "  the  small  stand  is  to  be  preferred. 

And  this  leads  us  to  enquire  what  is  or  ought  to  be  meant  by 
"ordinary  work  ?"  The  expression  is  not  an  uncommon  one, 
but  it  is  one  to  which  objection  has  been  made  on  account  of 
alleged  indefiniteness. 

If  we  exclude  from  consideration  the  employment  of  the  micro- 
scope for  purposes  of  amusement  or  of  elementary  instruction, 
almost  all  microscopic  work  may  be  divided  into  two  classes, 
which  may  very  properly  be  called,  respectively,  ordinary  work 
and  purposes  of  research.  Ordinary  work  in  this  sense  consists 
chiefly. in  searching  for  known  forms,  and  does  not  demand 
anything  like  the  instrumental  perfection  which  is  required  for 
work  which  at  all  approaches  the  nature  of  original  research. 
And  by  original  research,  we  do  not  mean  that  research  which 
is  carried  on  for  the  purpose  of  making  important  scientific  dis- 
coveries, but  simply  that  which  has  for  its  end  the  examination 
of  the  structure  and  characteristics  of  objects  which  are  un- 
known to  the  individual  observer. 

In  support  of  this  view,  it  would  be  easy  to  cite  numerous 
instances.  Thus  every  one  knows,  that  for  the  examination  of 
different  kinds  of  starch,  and  the  investigation  of  their  peculiar- 
ities, very  good  microscopes  and  high  powers  ought  to  be  used. 
But  Hassal,  in  his  work  on  adulteration,  tells  us  that  a  quarter- 
inch  objective,  of  ordinary  quality,  and  used  on  a  common 
ptand,  is  sufficient  to  discriminate  between  the  different  starches 
when  they  are  mixed  together. 


OF  THE  MICROSCOPE. 

The  study  of  the  diatoms,  in  so  far  as  their  structure  is  con- 
cerned, confessedly  requires  microscopes  and  objectives  of  the 
very  highest  class.  And  yet  we  have  been  told  by  a  gentleman, 
whose  name  deservedly  stands  very  high  in  connection  with  the 
study  of  these  interesting  objects,  that  for  the  determination  of 
species,  and  for  most  other  points  which  he  has  studied,  he  has 
used  a  good  non-adjusting  quarter,  on  a  simple  stand,  and  that 
it  is  but  seldom  that  he  has  recourse  to  better  objectives,  of 
which,  however,  he  has  a  full  series  by  the  most  eminent 
makers. 

By  the  term  "  ordinary  work,"  then,  we  may  very  properly 
designate  all  such  examinations  as  lead  us  over  ground  that  is 
well  known,  while  no  work  that  involves  the  examination  of 
unfamiliar  structures  or  the  investigation  of  unknown  processes, 
can  be  so  called.  But  of  all  the  work  that  is  done  with  the  mi- 
croscope, four-fifths  (including  the  work  of  the  physician)  is 
"ordinary  work." 

To  those,  therefore,  who  have  abundant  means  at  their 
command,  we  would  by  all  means  recommend  the  pur- 
chase of  two  microscopes — always,  of  course,  getting  the  cheap 
one  first.  If,  during  the  course  of  their  "  apprenticeship,"  this 
cheap  instrument  should  get  injured,  the  loss  will  not  be  very 
great,  and  by  the  time  the  student  has  learned  to  use  the  cheap 
instrument,  he  will  have  acquired  intelligent  views  as  to  his 
special  needs  in  the  matter  of  a  more  complete  one,  and  will 
not  be  dependent  upon  the  advice  of  any  one. 

To  those  who  cannot  afford  two  microscopes,  we  would 
strongly  recommend  as  a  stand,  one  of  the  New  American 
Models,  previously  described.  If  we  except  a  few  special  de- 
partments, such  as  goniometry,  etc.,  these  stands  are  equal  to 
all  demands,  and  will  do  justice  to  any  objectives  or  accessories, 
while  at  the  same  time  they  are  sufficiently  compact  and  handy 
to  allow  of  the  convenient  and  rapid  accomplishment  of  all 
kinds  of  simple  work. 

To  those  who  cannot  afford  one  of  these  stands,  the  only 
advice  we  can  offer  is  to  get  the  best  they  can  for  the  amount  of 
money  they  have  to  spend. 

It  would  be  impossible  to  give  anything  like  a  list  of  special 
cases  in  which  the  different  styles  of  microscopes  prove  most 


100  SELECTION  AND    USE 

useful  :  the  reader  whose  attention  is  called  to  this  point  will 
have  little  difficulty  in  deciding  the  question  for  himself.  We 
merely  give  the  general  rule,  that  where  dissections  of  plants 
and  animals  are  to  be  carried  on,  a  simple  microscope  should 
in  general  be  chosen,  while  the  compound  microscope  fur- 
nished with  good  objectives,  is  indispensible  whenever  high 
powers  are  required  for  the  examination  of  objects. 

Having  decided  upon  the  kind  of  microscope  that  is  needed, 
the  next  step  is  to  determine  the  individual  quality  of  the  dif- 
ferent instruments  that  may  be  offered  to  us.  To  do  this 
thoroughly,  it  will  in  every  case  be  found  a  good  plan  to  take 
up,  point  by  point,  all  those  elements  that  are  necessary  or  desir- 
able in  a  microscope,  and  in  this  way  subject  the  instrument 
to  the  most  careful  scrutiny.  Unless  a  microscope  is  made 
specially  to  order,  it  will  be  difficult  to  find  one  that  will  com- 
bine all  desirable  features,  but  the  plan  we  suggest  certainly 
enables  us  to  decide  most  readily  and  accurately  as  to  the  pres- 
ence or  absence  of  those  points  which  are  desirable  for  our  pur- 
.poses.  The  following  are  the  chief  points  that  demand  atten- 
tion: 

Magnifying  Power.— We  place  this  first,  because  usually 
the  first  question  in  regard  to  a  microscope  that  is  asked  by  be- 
ginners is,  "  What  is  its  magnifying  power?  "  Now  magnifying 
power,  although  an  important  element,  is  after  all  but  a  secondary 
consideration.  A  microscope  magnifying  a  thousand  diame- 
ters could  easily  be  made  and  sold  at  a  profit  for  five  dollars, 
and  a  few  cents  expended  in  paper  and  paste  will  at  any  time 
double,  or  even  treble,  the  magnifying  power  of  an  ordinary 
compound  instrument.  The  proper  question  is  not  how  much 
does  a  microscope  magnify,  but;  how  much  will  it  show.  A 
magnifying  power  of  one  hundred  diameters,  obtained  by  the 
use  of  first-class  objectives,  will  enable  us  to  see  more  of  the 
true  structure  of  an  object  than  could  be  reached  by  a  magni- 
fying power  of  five  hundred,  the  lenses  in  the  latter  case  being 
of  inferior  quality.  But,  although  not  the  first  consideration, 
11  agnifying  power  is  a  feature  of  sufficient  importance  to  deserve 
care  ul  deliberation,  and  without  a  knowledge  of  the  powers 
required,  and  the  mode  in  which  they  are  expressed,  the  begin- 


OF    THE  MICROSfCOP^.  101 

ncr  will  often  encounter  difficulty-  BoXli  tuese  pcinfc  l>?fng  es- 
sential, therefore,  before  disctissiag*  tlie 'migi«ify;r.gL 'powers 
best  suited  to  different  purposes,  it  may  be  well  to  say  a  word  in 
regard  to  the  mode  in  which  magnifying  power  is  always  ex- 
pressed by  scientific  men. 

When  we  look  at  a  small  object  through  a  microscope,  and 
see  it  magnified  to  twice  its  length,  it  is  evident  that  its  breadth 
is  also  magnified  twice,  and  consequently  its  surface,  no  matter 
what  the  shape  may  be,  is  magnified  four  times.  It  might  also 
be  said  that  as  we  only  take  cognizance  of  bodies  having  a  sen- 
sible thickness,  this  thickness  must  be  magnified  twice,  and 
therefore  the  object  is  magnified  twice  four,  or  eight  times.  The 
latter,  however,  is  a  view  which  is  never  insisted  upon,  and 
even  those  who  claim  the  most  for  their  microscopes,  never  do 
more  than  express  the  magnifying  power  in  surfaces.  Scien- 
tific men  are,  however,  agreed  tliat  to  express  a  magnifying 
power  in  surfaces  is  to  convey  a  wrong  impression  in  regard 
to  the  assistance  rendered  by  the  instrument  to  the  natural 
vision,  for  a  careful  study  of  the  physiology  of  vision,  teaches  us 
that  our  power  to  appreciate  and  distinguish  the  features  of 
any  object  depends  upon  the  distances  to  which  the  characteris- 
tic points  of  that  object  are  separated,  and  this  can  be  meas- 
ured only  by  linear,  and  not  by  superficial  units.  There  are 
other  considerations  which  lead  to  the  same  conclusion,  but 
for  the  beginner  it  is  sufficient  to  know  that  all  scientific  mi- 
croscopists  are  agreed  that  when  the  magnifying  power  of  a 
microscope  is  stated,  it  shall  be  stated  in  diameters,  and  not  in 
areas.  By  common  consent,  then,  ten  times  means  ten  diame- 
ters. And  yet  it  is  a  very  common  thing  for  charlatans,  and 
those  who  wish  to  deceive  the  public,  to  say  that  a  microscope 
sold  by  them  magnifies  ten  thousand  times,  or  one  hundred  di- 
ameters, and  as  "  ten  thousand  times  "  is  much  more  readily 
appreciated  by  the  popular  mind  than  "one  hundred  diame- 
ters," the  majority  of  those  who  read  such  statements  suppose 
that  they  will  be  enabled  to  see  ten  thousand  times  more  than 
they  could  see  with  the  naked  eye,  which  assuredly  is  not  the 
case.  In  some  instances  these  advertisers  do  not  even  state 
the  diameters.  We  have  now  before  us,  clipped  from  a  journal 
of  deservedly  good  reputation,  an  advertisment  which  reads  as 


102  ^  :£Et,ECTION    AND    USE 

follows^  Qmitt^ng  what  printer?  call  the  "display  "  arrangement 
°p|  tih^vdjtfa:  M  Microscopes  constructed  on  scientific  principles 
magnifying  10,000  times."  The  microscope  in  question,  as  we 
learned  by  j>ersonal  examination,  gives  a  magnifying  power  of 
about  one  hundred  diameters.  Carpenter  speaking  upon  this 
point  says:  "  The  superficial  magnifying  power  is  of  course  es- 
timated by  squaring  the  linear  ;  but  this  mode  of  statement  is 
never  adopted  by  scientific  observers,  although  often  em- 
ployed to  excite  popular  admiration,  or  attract  customers,  by 
those  whose  interest  is  concerned  in  doing  so. "  We  would, 
therefore,  advise  our  readers  to  look  with  suspicion  upon  any 
concern  advertising  in  this  manner.  Of  course  an  advertisement 
claiming  a  magnifying  power  of  "10,000  areas  or  100  diame- 
ters "  is  unobjectionable,  because  both  expressions  are  placed 
upon  an  equal  footing.  It  must  also  be  borne  in  mind  that 
great  though  unintentional  mistakes  are  often  made  by  dealers 
in  stating  the  power  of  the  microscopes  they  offer  for  sale. 
Not  long  ago  a  friend  told  us  that  he  had  been  offered  a  small 
microscope  having  a  magnifying  power  of  500  diameters,  for 
a  moderate  sum.  We  called  to  see  it,  taking  the  precaution  to 
put  a  micrometer  and  a  foot  rule  in  our  pocket.  By  actual 
measurement  the  highest  magnifying  power  of  this  microscope 
was  45  diameters  !  Another  instance  occurs  in  the  catalogue 
of  a  well-known  and  honorable  business  house,  who  offer  a 
very  neat  and  well  made  instrument,  whose  magnifying  power 
is  claimed  to  be  350  diameters.  Careful  measurement  of  several 
instruments,  however,  gave  an  average  power  of  less  than  200 
diameters!  Indeed  it  will  in  general  be  found  that  the  magni- 
fying power  stated  by  dealers  who  do  not  devote  their  chief  at- 
tention to  microscopes,  is  greatly  over  estimated. 

So  much,  then,  being  clearly  understood  in  each  case,  the 
question  naturally  arises,  What  should  be  the  magnifying  powers 
possessed  by  microscopes  intended  for  certain  specified  purposes? 
That  a  certain  magnifying  power  is  necessary,  no  matter  what 
the  quality  of  the  lenses  may  be,  is  true  beyond  a  doubt. 
Thus,  for  example,  suppose  we  wish  to  see  the  lines  on  the 
Pleurosigma  Angulatum,  which  lines  are  about  the  one  fifty- 
thousandth  of  an  inch  apart;  what  magnifying  power  would 
be  necessary? 


OF    THE   MICKOSCCCPE.  103 

With  the  best  illumination,  the  average  human  eye  can  just 
clearly  distinguish  lines  which  are  the  twd-hundredth  of  an  inch 
apart.  Some  eyes,  under  favorable  circumstances,  can  see 
lines  placed  as  close  together  as  250  to  the  inch,  but  the  aver- 
age is  as  we  have  stated.*  To  be  visible  even  to  the  best  eyes, 
therefore,  the  lines  on  the  Angulatum,  must  be  magnified  so 
that  they  will  present  the  same  appearance  as  lines  spaced  so  as 
to  give  at  the  very  most,  say,  200  to  the  inch.  This  requires  a 
magnifying  power  of  250  diameters,  and  with  less  than  this 
they  cannot  be  seen,  no  matter  how  good  the  objective  may  be. 
And  when  Dr.  Frey  says  that  they  can  be  seen  with  a  power  of 
80  or  100  times,  while  "weaker  objectives,  magnifying  40  or  50 
times,  should  show  something  of  the  lines,"  he  makes  a  state- 
ment that  we  cannot  accept. 

In  order,  therefore,  that  an  object  may  be  distinctly  seen,  it 
must  be  magnified  to  a  certain  extent,  but  the  magnifying 
power  absolutely  necessary  in  any  given  case,  will  also  depend 
upon  whether  the  microscope  is  to  be  used  for  general  purposes 
of  investigation,  or  merely  for  the  recognition  of  known  forms. 
For  the  latter  purpose  a  power  of  100  may  be  sufficient,  while 
for  the  former,  on  the  same  class  of  objects,  a  power  of  500 
would  be  the  least  that  would  be  serviceable.  The  following 
are  a  few  of  the  cases  in  which  the  power  required  can  be  stated 
approximately : 

For  medical  purposes  (except  for  pocket  instruments,  intended 
merely  to  enable  the  observer  to  recognize  known  forms)  a 
power  of  400  is  needed,  and  the  objective  should  be  of  really 
excellent  quality. 

Students  of  histology  require  a  microscope  with  a  wider 
range  of  power.  Low  powers  are  more  useful  to  them  than 
to  the  medical  man,  and  if  they  push  their  researches  in  cer- 
tain directions,  there  is  no  limit  to  the  magnifying  power  needed. 

*To  test  the  statement  in  the  text,  place  a  glass  micrometer,  ruled  200 
lines  to  the  inch,  on  the  stage  of  a  microscope,  and  by  means  of  the 
mirror  throw  a  beam  of  light  upon  it,  just  as  if  for  examination  by  trans- 
mitted light  in  the  usual  way.  If  we  now  look  at  the  lines,  not  through 
the  tube,  but  simply  from  one  side,  they  will  appear  distinctly  as  well-de- 
fined lines.  Try  the  same  with  a  micrometer  ruled  250  to  the  inch;  some 
eyea  will  be  able  to  distinguish  the  lines,  but  very  many  will  fail  to  do  s  \ 


104  .SELECTION   AND    US3 

A  good  two-third,  one-fifth,  and  one-tenth,  giving  magnifying 
powers  of  from  50  to  1000  diameters,  will,  in  general,  answer 
most  requirements.  It  must  be  borne  in  mind,  however,  that 
beginners  can  hardly  be  expected  to  use  a  one-tenth  inch  objec- 
tive to  great  advantage,  and,  therefore,  the  purchase  of  this 
item  may  safely  be  deferred. 

For  the  study  of  botany,  and  the  ordinary  facts  of  vegetable 
physiology,  a  power  of  300  is  sufficient;  but  the  very  minute 
forms  of  vegetable  life  require  a  much  higher  power,  and  so 
do  certain  of  the  higher  points  in  the  physiology  of  plants. 

For  the  detection  of  adulteration,  Hassal  recommends  the 
inch  and  the  quarter-inch  objectives,  giving  a  magnifying  power 
with  No.  1  and  No.  2  eye-pieces,  of  from  60  to  350  diameters. 
For  ordinary  purposes  of  instruction  and  amusement  in  the 
household,  a  microscope  magnifying  from  30  to  150  diameters 
will  be  found  most  satisfactory,  and  for  these  reasons:  Such  an 
instrument  is  easily  managed ;  if  well  made  it  gives  a  power 
amply  sufficient  for  all  ordinary  objects,  and  it  need  not  be  ex- 
pensive. Moreover,  while  it  is  an  easy  matter  to  prepare  ob- 
jects so  that  they  maybe  seen  satisfactorily  under  low  and  me- 
dium powers,  it  requires  great  skill  and  long  practice  to  enable 
tlie  student  to  prepare  objects  so  that  they  may  be  examined 
with  profit  under  a  high  power.  And  finally,  under  a  high  power, 
but  a  very  small  portion  of  any  ordinary  object  can  be  seen  at 
once,  and  consequently  many  of  those  things  that  are  best 
suited  for  popular  examination  can  only  be  seen  piecemeal — 
a  very  unsatisfactory  mode  of  proceeding.  Thus,  under  a  power 
of  750  diameters,  a  fly's  foot  could  not  possibly  be  seen  as  a  Avhole ; 
we  might  examine  a  single  claw  or  pad  at  a  time,  but  not  the 
whole  foot,  and  consequently  would  find  great  difficulty  in  ac- 
quiring an  idea  of  what  the  general  structure  of  the  foot  is. 
To  give  the  reader  clearer  ideas  upon  this  point,  we  have  just 
measured  the  diameters  of  the  fields  seen  under  French  and 
American  objectives,  with  the  following  results:  With  a  magni- 
fying power  of  25  diameters,  the  field  is  about  a  quarter  of  an 
inch;  with  50  diameters,  it  is  one-eighth  of  an  inch;  with  100 
diameters,  one-sixteenth  of  an  inch;  with  500  diameters,  one- 
eightieth  of  an  inch;  and  with  1000  diameters,  the  one-hundred- 
of  au  inch,  a  space  which  is  ordinarily  invisible 


OF    THE   MICROSCOPE.  105 

to  the  naked  eye.  Consequently,  when  these  high  powers  are 
used,  it  becomes  very  difficult  for  beginners  to  place  the  object 
properly  under  the  microscope,  for,  as  will  be  readily  seen,  unless 
it  is  adjusted  with  a  variation  less  than  the  one-hundred-and- 
fiftieth  of  an  inch,  it  cannot  be  seen  at  all. 

The  lowest  powers  that  will  show  satisfactorily  certain  well- 
known  objects,  are  about  as  follows:  The  scales,  or  so-called 
feathers  on  the  wings  of  most  butterflies  car  Ko  very  well  seen 
with  a  power  of  25  diameters;  under  the  same  power,  the  eye 
of  a  fly  shows  very  distinctly  the  several  smaller  eyes,  or  ocelli, 
of  which  it  is  composed;  the  individual  corpuscles  or  globules 
of  the  frog's  blood  can  be  distinguished  with  a  power  of  about 
35  diameters,  human  blood  requiring  40  to  50;  to  show  dis- 
tinctly the  form,  etc.,  of  these  same  corpuscles  requires  a  power 
of  200  and  upwards.  The  same  may  be  said  of  starch  granules. 
Human  hair  and  wool  may  be  seen  veiy  satisfactorily  under  a 
power  of  100  diameters,  the  former  appearing  like  a  cord, 
a  quarter  of  an  inch  thick.  In  order  to  show  the  peculiar  char- 
acteristics of  these  fibres,  however,  the  lenses  must  be  good. 
Cotton  and  flax  can  be  readily  distinguished  under  a  power  of 
80  diameters. 

A  question  very  frequently  asked  in  regard  to  cheap  micro- 
scopes is,  Will  they  show  the  animalcules  in  water?  And  in 
almost  all  the  advertisements  of  cheap  microscopes,  we  are  told 
that  they  will  do  this.  Now,  good  well  water  does  not  contain 
animalcules  that  can  be  seen  with  ordinary  microscopes.  It 
is  only  in  stagnant  water  that  they  are  found,  and  many  of 
them  can  be  seen  with  the  naked  eye,  without  the  use  of  any 
microscope  whatever.  Others  require  the  use  of  microscopes 
having  powers  a  hundred  fold  greater  than  that  of  the  best 
microscopes  in  ordinary  use.  It  is  evident,  therefore,  that 
such  statements  are  worthless  as  affording  any  indication  of 
the  character  of  a  microscope.  A  microscope  magnifying  fif- 
teen to  twenty  diameters  will  show  objects  that  are  perfectly 
invisible  to  the  naked  eye,  and  with  fifty  diameters,  provided 
the  definition  is  good,'  we  can  obtain  a  very  interesting  view  of 
many  of  the  most  beautiful  objects  described  in  the  books,  and 
sometimes  called  animalcules,  such  as  the  Volvox  Globator,  the 
larger  Vorlicelliy  etc.,  etc. 


106  SELECTION    AND    USE 

Tlte  Stand. — This  should  be  firm  and  substantial,  with,  the 
centre  of  gravity  very  low.  Nothing  detracts  so  much  from  the 
performance  of  an  objective  as  tremor  and  vibration,  an  da  largo 
majority  of  the  microscopes  in  market  are  very  shaky,  from  the 
fact  that  they  a-e  made  tall  and  showy  in  order  to  command  a 
higher  price.  It  is  well,  therefore,  to  bear  in  mind  that  size  is 
no  criterion  of  the  value  of  a  microscope.  Instrument  makers 
very  properly  give  the  size  of  their  instruments,  and  it  generally 
happens  that  the  largest  instruments  by  the  same  maker  bear 
the  highest  prices.  Other  things  being  equal,  however,  small, 
compact  instruments  are  altogether  to  be  preferred.  Some 
years  ago  the  rage  was  for  large,  showy  microscopes,  which 
made  a  fine  appearance  in  the  office  of  the  physician,  and  the 
study  of  the  naturalist.  It  was  found,  however,  that  in  this 
case  efficiency  was  sacrificed  to  show,  and  all  our  best  makers 
are  now  catting  down  the  sizes  of  their  instruments,  and  making 
them  steady,  substantial,  durable  and  easily  operated. 

There  is,  of  course,  a  limit  to  the  extent  to  which  stands  may 
be  reduced  in  size  without  sacrificing  their  efficiency,  and  some 
makers  seem  to  forget  this.  There  are  stands  iu  market  that 
are  too  small  every  way  for  anything  but  special  classes  of 
work.  The  bodies  are  too  small  to  secure  efficiency  in  the  eye- 
pieces and  objectives;  the  stage  is  too  small  to  allow  of  the  use 
of  slides  of  proper  size,  and  there  is  no  room  beneath  the  stage 
for  the  attachment  of  proper  illuminating  apparatus.  All  this 
is  as  inconvenient  as  the  three-feet-high  microscopes  of  the  end 
of  the  last  century. 

The  weight  of  the  stand  is  a  subject  concerning  which  many 
seem  to  differ  in  opinion.  One  writer  goes  so  far  as  to  say  that 
no  stand  weighing  less  than  fifteen  pounds  can  be  steady  enough 
for  the  performance  of  good  work.  It  will  be  found,  however, 
that  a  judicious  distribution  of  the  material,  and  a  proper  con- 
struction of  the  different  parts,  will  more  effectually  resist  the 
usual  sources  of  unsteadiness  than  any  increase  of  absolute 
weight.  Of  course,  if  it  is  merely  desired  to  make  the  micro- 
scope steady,  in  the  sense  that  an  inkstand  is  steady — that  is, 
not  liable  to  be  tipped  over — weight  is  everything.  But  the 
stands  that  are  most  difficult  to  tip  over  are  not  those  that 
resist  vibrations  most  perfectly.  For  the  latter  a  tripod  with  a 


OF  THE  MICBOSCOPE.  107 

small  area  of  support  is  best;  for  the  former  a  stand  with  a  flat 
base  resting  over  its  whole  surface  on  the  table  should  be  pre- 
ferred. 

It  is  obvious  that  the  causes  of  unsteadiness  are  either  vibra- 
tions transmitted  from  the  floor,  or  movements  caused  by  the 
hand  in  performing  the  necessary  manipulations  and  adjust- 
ments. The  first  can  never  be  stopped  by  weight,  unless, 
indeed,  we  make  the  stand  so  heavy  that  its  weight  will  impart 
rigidity  to  the  table  and  floor,  and  this  would  require  a  good 
deal  more  than  fifteen  pounds,  or  even  twicer  that.  For  the 
checking  of  vibrations  transmitted  from  the  floor,  no  device  is 
better  than  the  stand  or  table  described  in  a  subsequent  sec- 
tion. So  far  as  movements  transmitted  by  the  hand  are  con- 
cerned, if  a  stand  of  three  or  four  pounds  will  not  resist  them, 
the  observer  should  set  himself  about  learning  delicacy  of 
movement  before  he  proceeds  any  further. 

All  microscopes  made  in  tliis  country  and  in  England  are 
now  constructed  so  that  the  body  may  be  inclined  to  any  angle, 
thus  giving  the  power  of  using  the  microscope  in  any  position 
— vertical,  inclined  or  horizontal.  The  importance  of  this  is 
easily  seen  when  we  consider  that  on  the  one  hand,  when  liquids 
are  to  be  examined,  it  is  sometimes  necessary,  or  at  least  desira- 
ble to  use  the  microscope  in  a  vertical  position,  though  this  is 
a  very  tiresome  and  inconvenient  position,  and  one  that  is  not 
calculated  to  enable  the  observer  to  obtain  the  best  possible  re- 
sults; and  on  the  other,  it  is  equally  necessary  that  the  body  of 
the  microscope  should  be  capable  of  assuming  the  horizon- 
tal position  when  the  camera  lucida  is  to  be  employed  for 
making  drawings,  as  will  be  hereafter  explained.  And  yet 
Frey  actually  gives  the  preference  to  microscopes  that  do  not 
incline,  and  which  must  always  be  used  in  a  vertical  position ! 
This,  of  course,  necessitates  the  complicated  and  expensive  ar- 
rangement which  he  describes  for  adapting  the  camera  lucida  to 
the  vertical  instrument,  a  singular  instance  of  prejudice  against 
an  obvious  and  successful  improvement. 

T lie  Stage.— In  every  case,  a  large,  roomy  stage  is  of  the 
utmost  importance.  One  great  objection  to  most  French  in- 


108  SELECTION  AND    USE 

struments  is  that  the  stages  are  too  small.  It  should  also  be 
firm  and  substantial,  so  that  its  position  in  regard  to  the  other 
parts  of  the  stand  cannot  be  varied  by  slight  pressure.*  The 
most  important  points  connected  with  the  stage  are  the  means 
provided  for  holding  and  moving  the  object,  and  the  facilities 
afforded  for  attaching  accessory  apparatus. 

In  the  most  complete  stands,  the  object  is  held  between 
sliding  clips,  which  form  a  sort  of  clamp  that  is  capable  of 
being  moved  in  two  directions,  at  right  angles  to  each  other, 
by  mechanical  means,  which  generally  consist  of  a  screw  for 
one  direction  and  a  rack  and  pinion  for  the  other.  This  form, 
which  is  known  as  the  mechanical  stage,  enables  even  a  com- 
paratively unskilled  person  to  bring  any  part  of  the  object  into 
the  desired  position  in  the  field  of  view,  and  this  with  the 
utmost  precision.  These  mechanical  stages  may  be  said  to  be 
characteristic  of  the  higher  classes  of  English  microscopes,  and 
as  they  are  expensive,  they  are  not  generally  used.  Neither 
are  they  absolutely  necessary  for  ordinary  work  with  low  or 
medium  powers,  for  with  any  objective  lower  than  one-twelfth 
of  an  inch  focus,  the  object  can  be  moved  by  hand  quite  as 
readily  as  by  the  screws,  and  we  hold  it  to  be  a  well  established 
rule  in  all  manipulations  connected  with  scientific  work,  that 
whenever  any  operation  can  be  performed  satisfactorily  by 
means  of  the  hands  alone,  all  special  contrivances  should  be 
dispensed  with.  For  low  and  moderate  powers,  therefore,  we 
prefer  the  plain  stage,  on  which  the  object  is  moved  by  means 
of  the  hands  alone.  But  when  very  high  powers  are  used,  and 
especially  when  delicate  micrometrical  or  goniometrical  meas- 
urements are  to  be  made,  a  well-made  mechanical  stage  becomes 
a  necessity.  For  while  it  is  easy  enough  to  bring  an  object 
very  near  to  a  given  point  by  means  of  the  fingers  alone,  it  is 
almost  impossible  to  secure  perfect  accuracy.  In  the  effort  to 
attain  this  the  mechanical  stage  is  a  great  assistance,  and 
therefore  when  Frey  utters  a  wholesale  condemnation  of  the 

*At  the  same  time,  however,  it  must  be  borne  in  mind  that  no  stage 
ever  was  made  so  firm  that  even  a  slight  pressure  would  not  affect  if.  If, 
therefore,  the  reader  is  determined  not  to  rest  content  with  anything 
short  of  a  perfectly  rigid  stage,  he  will  reject  all  the  best  microscopes  in 
market. 


*  OF  THE  MICROSCOPE.  109 

'  English  microscopes,  and  asserts  that  they  are  unpleasantly 
loaded  with  what  he  is  pleased  to  call  "screws  and  unessenti  il 
appurtenances,"  it  seems  to  us  that  he  commits  a  great  error. 
These  costly  and  complex  instruments  are  intended  for  the 
highest  class  of  work,  and  the  most  powerful  objectives;  per- 
fection of  the  work  to  be  done,  and  not  simplicity  in  the  means 
by  which  it  is  to  be  done,  is  the  end  sought,  and  this  can  be 
attained  only  by  the  complex  means  employed. 

We  have  never  found  any  of  the  so-called  lever  stages  that 
fulfilled  the  requirements  of  the  highest  class  of  work,  and, 
therefore,  if  a  mechanical  stage  is  to  be  chosen  at  all,  the  best- 
form  should  be  procured. 

A  microscope  fitted  with  a  good  mechanical  stage  leaves 
nothing  to  be  desired,  but  when  other  forms  are  used,  it  is  evi- 
dent that  the  chief  points  to  be  attained  are  these:  1.  The 
object  should  be  held  steadily,  but  at  the  same  time  perfect 
freedom  of  motion  should  be  allowed.  2.  Ifc  should  be  possi- 
ble to  remove  instantly  from  the  surface  of  the  stage,  every- 
thing in  the  shape  of  clips  and  holders,  so  that  a  clear  field 
should  be  left  for  the  adjustment  of  very  large  slides,  plates, 
etc.,  or  for  the  rotation  of  the  object  in  relation  to  the  light. 
3.  Even  the  simplest  forms  of  the  stage  should  be  so  constructed 
that  it  may  be  possible  to  pass  every  part  of  the  object  under 
the  field  of  view,  and  this,  without  any  risk  of  omitting  even 
the  smallest  portion.  This  point  is  of  special  importance  to 
physicians  and  naturalists.  Thus,  it  not  unfrequently  happens 
that  it  is  desirable  to  know  whether  or  not  certain  forms  are 
present  in  a  given  drop  of  liquid;  unless  we  can  subject  every 
part  of  that  drop  to  microscopical  examination,  we  cannot  be 
sure  that  the  forms  we  are  looking  for  are  absent.  There  is 
always  a  risk  of  omitting  some  portion  of  the  slide,  and  conse- 
quently doubt  must  always  hang  over  the  exhaustiveness  of  all 
our  examinations.  The  only  certain  means  of  avoiding  all  risk 
of  missing  any  portion  of  a  given  slide  is  to  pass  it  across  the 
field  of  view  in  successive  parallel  bands,  just  as  a  plowman 
plows  a  field.  The  process  is  clearly  shown  in  the  diagrams 
on  the  following  page.  Fig.  22,  showing  the  mode  in  which  the 
entire  surface  is  completely  covered  with  a  series  of  parallel  rib- 
bons, the  breadth  of  each  of  which  is  the  diameter  of  the  field 


110  SELECTION  AND  USE 

of  view,  while  Fig.  23  shows  the  hap-hazard  way  in  which  ex- 
aminations are  usually  made,  abundant  room  being  left  (as 
shown  by  the  small  crosses)  for  the  escape  of  important  fea- 
tures, Now,  with  ordinary  clips,  it  is  difficult  to  effect  this, 


Fig.  22.  Fig.  23. 

although  it  may  be  done  by  the  aid  of  a  plate  of  metal  or  vul- 
canite, a  little  thicker  than  the  glass  object-slide,  and  having  a 
straight  edge.  The  forward  movement  is  made  by  pushing  the 
vulcanite  plate,  while  by  sliding  the  object  along  the  edge  of 
this  plate,  we  are  enabled  to  examine  a  narrow  strip,  the  width 
of  the  field  of  view,  as  shown  in  Fig.  22.  A  somewhat  similar 
device  is  shown  on  the  stage  of  Zentmayer's  Histological  Micro- 
scope, Plate  IV.  Wlion  slightly  modified,  this  device  also 
serves  as  a  guide  for  the  Maltwood  finder. 

In  the  simpler  forms  of  the  stage,  the  object  is  held  in  place 
by  spring  clips,  which  press  it  down,  and  under  which  it  is 
moved.  These  clips  are  frequently  screwed  to  the  stage,  which 
is  a  great  mistake,  as  we  are  thus  prevented  from  slipping  them 
off,  so  as  to  leave  the  stage  entirely  clear.  They  should  always 
be  held  in  such  a  way  that  they  may  be  instantly  removed,  and 
they  should  also  be  very  thin  and  springy,  otherwise  it  is  im- 
possible to  move  the  object  with  sufficient  delicacy. 

The  so-called  glass  stage,  or  Zentmaycr  stage,  has  come  into 
extensive  use,  and  is  very  much  liked  by  some.  As  made  by 
Zentmayer,  it  consists  of  a  plate  of  glass,  held  against  a  brass 
support  by  means  of  a  spring,  which  is  pointed  with  ivory. 
The  friction  of  the  glass  plate  upon  its  metal  support  is  thus 
easily  adjusted,  and  may  be  either  so  reduced  that  the  plate  will 
respond  to  the  slightest  touch,  or  it  may  be  so  firmly  clamped 
as  to  be  practically  immovable. 


OF  THE   MICROSCOPE.  Ill 

In  some  forms  the  glass  plate  is  stationary,  and  the  object- 
carrier  moves  over  it,  the  points  of  contact  being  very  small. 


Revolving  Stage.— It  is  often  desirable  to  rotate  an 
object  in  .the  optic  axis  of  the  microscope,  either  for  the  pur- 
pose of  measuring  angles  or  changing  the  direction  of  the 
illumination  in  regard  to  the  object.  Means  for  effecting  this 
with  perfect  accuracy  have  been  applied  both  to  the  mechanical 
stage  and  the  glass  stage,  though  the  latter  is  generally  so  con- 
structed that  rotation  is  impossible.  In  the  large  Koss  stand  (PI. 
I),  the  stage  rotates,  and  is  graduated  for  measuring  angles. 

M.  Xachet  has  devised  a  special  form  of  the  glass  stage,  in 
which  provision  is  made  for  rotating  it.  In  this  form  of  the 

stage  the  object-earner,  F,  is 
held  to  the  glass  plate,  E,  by 
means  of  two  springs  with  ivory 
points,  the  springs  being  at- 
tached to  the  frame  in  which 
the  glass  plate,  E,  is  set.  Both 
the  plate,  E,  and  the  object- 
carrier  rotate  in  a  light  brass 
frame.  In  all  these  cases  an 
attempt  is  made  to  secure  coin- 
cidence of  the  centre  of  rota- 
tion with  the  optic  axis  of  the 

instrument.  It  will,  however,  be  found  that  it  is  an  impossi- 
bility to  attain  this  object  with  any  great  degree  of  accuracy 
unless  provision  is  made  for  occasionally  adjusting  the  centre 
of  rotation  of  the  stage.  No  stage  that  ever  was  made  will  re- 
main for  any  length  of  time  so  true  that  angles  can  be  correctly 
measured  by  it,  and  therefore  several  devices  have  been  sug- 
gested for  securing  accuracy  without  adjusting  the  stage,  since 
when  Avell-made  an  adjustable  stage  is  expensive,  and  when 
poorly  made  it  is  worthless. 

The  best  known  of  these  devices  is  what  is  called  the  center- 
ing nose-piece.  This  is  a  kind  of  adapter  which  is  screwed  on 
the  body  and  receives  the  objective.  It  is  provided  with  center- 
ing arrangements,  and  the  objective  having  been  screwed  into 


112  ,    .  SELECTION  AND   USE 

it,  the  latter  may  now  be  adjusted  exactly  over  the  centre  of 
rotation  of  the  stage.  With  common  objectives  this  answers 
very  well,  but  first-class  objectives  are  apt  to  show  the  effect 
of  being  thrown  out  of  centre. 

Where  rotation  in  regard  to  the  illumination  alone  is  required, 
several  plans  may  be  employed.  One  of  the  oldest  is  that 
found  in  the  large  microscopes  of  Hartnack  and  others,  and 
recently  adopted  by  Mr.  Browning.  This  device  consists  in 
forming  the  stage  in  two  pieces,  the  lower  part  being  firmly 
connected  with  the  foot,  and  the  means  of  illumination,  while 
the  upper  part,  which  rotates  on  the  lower,  is  rigidly  attached  to 
the  arm  which  carries  the  body.  In  this  way  the  body,  the  arm, 
the  upper  part  of  the  stage  and  the  object  may  all  be  simultane- 
ously rotated  in  relation  to  the  illumination,  and  for  ordinary 
purposes  this  answers  very  well. 

When  polarized  light  is  used,  however,  it  is  obvious  that  it 
is  impossible  to  rotate  the  object  in  relation  to  the  polarized 
ray,  without  also  rotating  the  analyzer,  and,  as  every  one  that 
has  worked  much  in  this  direction  knows,  it  is  often  of  great 
importance  that  both  polarizer  and  analyzer  should  be  kept 
stationary,  while  the  object  itself  rotates  between  them. 

The  little  diatom  stage  devised  by  the  author,  and  shown  in 
Figure  26,  page  114,  also  affords  very  simple  and  convenient 
means  for  rotating  objects  either  above  or  below  the  stage. 
And  it  would  be  a  very  easy  matter  to  so  construct  it,  that  its 
centre  of  rotation  might  be  accurately  adjusted  to  the  optic 
axis  of  the  instrument. 

Hot  Stage.— It  is  frequently  desirable  to  keep  certain  pre- 
parations at  a  given  temperature  or  to  raise  them  considerably 
above  the  usual  temperature  of  the  atmosphere,  so  as  to  observe 
the  ac  ion  of  reagents  upon  them  and  the  effect  of  heat  upon 
their  vitality,  where  living  organisms  are  under  observation. 
Various  devices  have  been  worked  out  for  this  purpose.  A 
stage  consisting  of  two  plates,  each  with  a  central  aperture, 
and  so  united  as  to  form  a  tight  box  through  which  a  heated 
fluid  is  made  to  circulate,  is  probably  the  best.  For  tempera- 
tures under  212°  F.  water  is  the  best  liquid;  for  higher  tem- 
peratures oil  or  saline  solutions  may  be  used. 


OP  THE   MICEOSCOPE.  113 

For  beating  objects  under  the  microscope,  we  have  long 
used  a  thick  copper  wire  coiled  in  the  flat  so  as  to  lie  on  the 
stage  and  with  a  projecting  end  which  may  be  heated  by  a 
Jamp.  It  is  not  difficult  to  adapt  a  small  thermometer  so  as  to 
indicate  very  nearly  the  temperature  to  which  the  object  is 
subjected.  To  regulate  the  degree  of  heat  employed  we  pass 
the  end  of  the  wire  through  a  copper  tube  which  is  made  to  act 
as  a  chimney  to  the  lamp,  and  by  moving  this  tube  in  or  out, 
the  coil  on  which  the  object  lies  may  be  made  more  or  less  hot. 

Stages  for  Special  Purposes. — It  may  be  safely  asserted 
that  there  has  never  yet  been  constructed  a  stage  which  would 
suit  the  requirements  of  every  worker  with  the  microscope. 
Indeed,  each  investigator  seems  to  require  special  modifications 
of  his  own.  Thus,  it  will  be  found  that  the  ordinary  stage, 
with  all  its  appurtenances,  is  too  thick  to  admit  the  use  of  that 
very  oblique  illumination  which  is  required  by  the  worker  on 
diatoms,  while  if  the  stage  be  made  thin  enough  it  loses  the 
necessary  rigidity.  Some  makers  have  sought  to  obviate  this 
by  supplying  two  stages — a  stout  one  for  common  work,  and  a 
thin  one  for  diatoms.  A  microscope  now  in  our  possession  is 
furnished  with  an  extra  thin  stage,  which,  by  a  very  simple 
and  ingenious  device,  can  be  instantly  substituted  for  the  heavy 
one.  The  nricroscope  is  said  to  have  been  ,nade  by  Spencer  or 
Tolles,  and  must  have  been  made  about  the  year  1856,  or  even 
earlier.  Thin  stages,  on  the  same  principle,  called  Diatom  Stages, 
have  been  recently  introduced  by  several  makers,  thus  affording 
another  illustration  of  ,:fehe  aphorism  that  history  repeats  itself. 

The  same  object  is  also  attained  by  means  of  the  secondary 
stage,  invented  by  Mr.  Lewis  Eutherfurd.  This  is  simply  a 
skeleton  stage,  which  is  placed  on  the  ordinary  stage,  and  is 
raised  so  far  above  it  that  the  illumination  may  be  applied 
between  them.  Kays  of  great  obliquity  may  tlms  be  passed 
through  the  object.  Kutherfurd's  skeleton  stage  forms  also  an 
admirable  safety  stage,,  since  the  object,  being  held  against  the 
under  side  of  the  skeleton  stage,  yields  to  the  slightest  pressure 
of  the  objective.  Mr..  Spencer  has  also  taken  advantange  of  this 
principle,  and  so  formed  the  under  side  of  the  stage  in  some  of 
his  stands,  that  the  object  may  be  pressed  against  it  by  the 


114 


SELECTION"  AHi> 


clips,  which  for  this  purpose  are  pushed  through  from  below 
upwards.  In  focussing,  the  objective  is  passed  through  the 
stage  if  necessary.  Great  obliquity,  and  perfect  safety  against 
breakage  of  the  object  by  the  objective,  are  secured.  When  the 
microscopist  is  using  valuable  slides,  costing  from  ten  to  one 
hundred  dollars,  the  latter  feature  is  one  of  great  importance. 

In  many  microscopes,  however — notably  those  of  English 
manufacture — the  under  side  of  the  stage  is  not  flat  and  even, 
so  that  a  slide  cannot  be  laid  against  it.  The  following  simple 
device  obviates  this  difficulty:  A  rectangular  plate  of  metal  is 

pierced  with  a  hole  of  the  same 
size  as  the  interior  diameter  of 
the  sub-stage  ring  of  the  micro- 
scope, and  in  this  hole  is  fastened 
a  tube,  which  just  fits  this  sub- 
stage  ring.  The  plate  is  provided 
with  two  light  spring  clips,  which 
hold  the  object  against  its  under 
side,  and  it  is  easy  to  see  that  this 
simple  contrivance,  which  is  shown 
in  Figure  26,  serves  three  very  im- 
portant functions:  1.  It  affords 
means  for  obtaining  light  of  great 
obliquity,  since  in  reality  it  forms 
a  stage  which  has  no  thickness  at 
all.  2.  It  serves  as  a  perfect  safety 
stage,  thus  enabling  us  to  avoid  all 

risks  not  only  to  valuable  slides,  but  to  still  more  valuabe  ob- 
jectives. 3.  It  enables  us  to  rotate  the  object  very  nearly  in 
the  optic  axis  of  the  instrument. 

The  centricity  of  this  rotation  is  not  sufficiently  accurate  to 
enable  us  to  measure  angles  with  any  degree  of  precision,  but 
it  gives  us  the  opportunity  of  placing  lined  diatoms  and  other 
objects  in  any  position  in  regard  to  the  illuminating  ray. 

Sub-Stage.— The  sub-stage  is  used  chiefly  for  holding  and 
adjusting  illuminating  apparatus  beneath  the  stage,  and  may 
justly  be  regarded  as  one  of  the  most  important  parts  of  the 
stand. 


Fig.  26. 


03?  THE  5IICKOSCOPE.  115 

Ifc  forms  a  prominent  feature  in  the  New  American  Model,  as 
well  as  in  all  first-class  microscopes. 

In  its  simplest  form  it  consists  simply  of  a  short  tube  or  ring, 
which  is  attached  to  the  under  side  of  the  stage,  and  is  fitted  to 
receive  polarizers,  paraboloids,  condensers,  etc.  It  should  be 
made  removable,  so  that  it  may  offer  no  obstacle  to  the  employ- 
ment of  the  most  oblique  illumination.  . 

In  the  New  American  Model  it  is  attached  to  a  swinging  arm, 
so  that  the  illuminating  apparatus,  which  it  carries,  may  be 
placed  at  any  angle  with  the  optic  axis  of  the  microscope. 

When  a  mere  ring  is  used  for  receiving  the  accessory  pieces 
of  apparatus,  the  latter  are  adjusted  by  simply  pushing  them 
up  or  down,  but  a  much  better  plan  is  to  have  the  ring  itself 
moveable,  as  is  the  case  in  the  instruments  shown  in  Plates  III, 
IV,  V,  and  VI.  The  accessory  is  then  placed  in  the  ring,  and 
the  latter  may  be  slid  up  or  down  the  carrying  bar,  so  as  to 
allow  of  proper  adjustment.  A  still  better  plan  is  that  shown 
in  Plate  I,  where  the  distance  of  the  sub-stage  from  the  stage 
may  be  adjusted  by  means  of  a  rack  and  pinion. 

In  any  case  the  sub-stage  itself,  or  the  apparatus  which  it 
carries,  should  have  facilities  for  accurately  centering  the 
various  pieces  of  illuminating  apparatus. 

Tlie  Mirror.—  The  mirrors  employed  for  illuminating 
microscopic  objects  are  either  plane  or  concave,  and  in  the 
better  class  of  instruments  both  kinds  are  provided,  while  the 
cheaper  forms  of  the  microscope  have  only  the  concave  mirror. 
The  plane  mirror  reflects  the  light  just  as  it  falls  on  it — that  is 
to  say,  divergent  rays  (as  from  a  lamp)  remain  divergent  after 
reflection,  and  parallel  rays  (as  sunlight  or  rays  from  the  bull's- 
eye  condenser)  remain  parallel.  The  concave  mirror,  on  the 
other  hand,  causes  parallel  rays  to  converge  and  meet  at  a  point, 
while  other  rays  are  rendered  either  less  divergent,  parallel  or 
convergent,  as  the  case  may  be.  It  is  very  important  that  the  sur- 
face of  the  mirror  should  be  accurately  formed,  and  therefore  in 
all  good  microscopes  they  are  made  of  glass,  which  has  been 
accurately  ground  and  polished.  Blown  or  cast  glass  will  not 
answer.  And  as  the  quality  and  quantity  of  the  light  is  greatly 
affected  by  the  reflecting  surface,  the  best  mirrors  are  silvered 


116 


SELECTION   AND   USE 


with  pure  silver,  instead  of  Avith  tlie  amalgam  of  tin  and 
mercury,  ordinarily  used.  The  mirror  should  be  so  hung  that 
it  may  throw  a  beam  of  great  obliquity  through  the  object.  In 
many  cases  this  is  absolutely  necessary,  and  even  for  ordinary 
work  it  is  of  great  advantage,  since  it  not  only  enables  us  to 
resolve  lined  objects,  but  to  secure  important  changes  in  the 
illumination  of  common  objects.  A  very  fair  dark  ground 
illumination  may  be  secured  if  the  light  is  so  oblique  that  none 
of  it  can  enter  the  object-glass  directly. 

The  mirror  should  not  only  be  hung  so  as  to  swing  to  any 
angle,  but  it  should  be  movable  on  the  mirror-bar  so  that  the 
rays  which  it  reflects  may  be  brought  exactly  to  a  focus  on  the 
object.  This  is  done  by  sliding  the  mirror  out  or  in,  according 
as  the  rays  are  more  or  less  divergent. 

The  concave  mirror  should  be  large,  so  that  it  may  collect 
plenty  of  light.  The  plane  mirror  may  be  small  without  much 
loss.  The  concave  mirror  is  frequently  used  for  the  illumina- 
tion of  opaque  objects,  as  when  large  it  concentrates  the  light 
very  strongly.  For  this  purpose  it  is  either  mounted  on  a 
separate  stand,  or  the  mirror-bar  is  so  hung  that  it  may  be 
turned  up  over  the  stage,  so  as  to  reflect  the  light  down  upon 
the  object. 

Tlie  Body.— The  only  points  connected  with  the  body  of 
the  microscope  which  require  consideration  are  its  diameter  and 
its  length,  and  these  must  of  necessity  vary  so  much  according 
to  the  purposes  to  which  the  microscope  is  to  be  applied,  that 
no  rule  can  be  laid  down.  Pocket  microscopes  are  of  necessity 
small ;  microscopes  intended  for  use  with  objectives  of  low 
power  and  large  angles,  must  have  a  large  diameter.  And  since 
the  distance  of  the  eye-piece  from  the  objective  affects  the  cor- 
rection of  the  latter,  it  has  been  found  necessary  to  adopt  a 
standard  length  of  body.  This  has  been  fixed  at  ten  inches  in 
this  country  and  in  England.  On  the  continent  of  Europe, 
eight  inches  is  the  length  that  has  been  adopted,  and  most  of 
the  continental  objectives  are  corrected  for  this  length  of  body. 
Provided  it  is  large  enough  to  take  the  new  broad-gauge  screw, 
the  diameter  is  not  of  very  great  importance  in  bodies  of 
moderate  length,  but  Beale  tells  us  that  in  his  long  tubes,  in- 


OF  THE   MICROSCOPE.  117 

tended  to  produce  great  magnifying  power,  a  diameter  of  two 
to  two-and-a-half  inches  was  found  to  be  absolutely  necessary 
to  prevent  the  glare  arising  from  internal  reflection.  An  inch 
and  a  quarter  is  a  good  size  for  ordinary  instruments.  Since  a 
very  long  body  is  inconvenient  when  the  microscope  is  used  in 
a  vertical  position,  the  best  instruments  are  furnished  with  a 

Draw-Tube,  whereby  the  length  of  the  body  may  be 
varied  at  pleasure.  As  explained  in  a  former  paragraph  (page 
20),  when  the  distance  between  the  eye-piece  and  the  object- 
glass  is  increased,  the  magnifying  power  is  increased  also. 
The  draw-tube,  therefore,  gives  us  the  means  of  varying  and 
adjusting  the  magnifying  power  of  the  microscope,  and  this  is 
sometimes  of  great  use.  Thus,  suppose  it  were  required  to 
draw  an  object  to  a  scale  magnified  exactly  one  hundred  diame- 
ters ;  it  might  be  impossible  to  procure  an  eye-piece  and  an 
objective  that,  with  a  fixed  length  of  body,  would  give  exactly 
this  amplification,  but  when  we  are  able  to  vary  the  magnify- 
ing power  by  changing  the  length  of  the  body,  it  is  easy  to  get 
at  it  exactly.  This,  however,  is  but  one  of  many  advantages 
afforded  by  the  draw-tube.  If  the  objective  be  good,  and  the 
eye-piece  not  very  high,  an  easy  and  very  satisfactory  way  to 
increase  the  magnifying  power  of  the  microscope  is  to  lengthen 
the  body  by  means  of  an  additional  tube,  which  may  even  be  made 
of  smooth  paper.  But  it  must  be  borne  in  mind  that  any  change 
in  the  distance  between  the  eye-piece  and  the  objective  affects 
the  corrections  of  the  latter,  just  as  a  change  in  the  thickness 
of  the  covering  glass  would  do.  Consequently,  unless  the  ob- 
jective has  a  considerable  range  of  correction,  it  may  be  impos- 
sible to  get  good  results  when  a  very  long  draw-tube  is  used. 
On  the  other  hand,  the  draw-tube  may  be  used  to  good  ad- 
vantage as  a  means  of  correcting  for  covering  glass  when  non- 
adjusting  objectives  are  used.  We  have  a  one-fifth  objective 
now  before  us,  with  which  we  can  see  clearly  the  lines  on  the 
P,  angulatum  on  the  balsum  Probe  Platte  when  the  draw-tube 
is  out,  but  when  the  tube  is  pushed  in,  the  view  is  foggy  and 
indistinct.  This  is  due  to  a  disturbance  of  the  corrections. 

The  insides  of  all  draw-tubes   and  bodies   should   be   well 
blackened.    When  bright  or  white  the  glare  greatly  injures  the 


118  SELECTION  AND   USE 

defining  powBr.  When  draw-tubes  or  long  eye-pieces  are  so 
arranged  that  they  rub  against  the  inside  of  the  tube  in  which 
they  are  inserted,  they  invariably  make  the  latter  bright  by 
friction.  They  should,  therefore,  always  slide  in  a  collar. 

It  is  always  well  to  have  the  lower  end  of  the  draw-tube  fur- 
nished with  the  Society  screw,  as  by  this  means  it  is  sometimes 
possible  to,  use  objectives  of  greater  working  distance  than 
could  otherwise  be  employed,  and  this  arrangement  also  affords 
facilities  for  the  use  of  amplifiers,  erectors,  etc. 

Adjustments  for  Focussing.— In  the  cheaper  forms  of 
the  microscope  the  adjustment  is  made  directly  by  hand,  one 
tube  sliding  within  another.  In  a  better  class  of  instruments 
the  objective  is  brought  nearly  into  position  by  sliding  the 
body  through  an  outer  tube,  and  then  the  final  adjustment  is 
made  by  means  of  a  screw  or  other  mechanical  means.  But  in 
all  the  best  microscopes,  the  coarse  adjustment,  as  it  is  called, 
is  made  by  means  of  a  rack  and  pinion,  while  the  fine  adjust- 
ment is  made  in  the  manner  just  mentioned.  Instead  of  a  rack 
and  pinion,  a  chain  is  sometimes  employed,  and  the  coarse 
adjustment  is  also  made  in  some  cases  by  screws  of  very  wide 
pitch,  and  similar  devices.  Nothing,  however,  can  equal  a 
smoothly  cut  and  well-fitted  rack  and  pinion.  It  is  sometimes 
alleged  that  the  chain  is  more  delicate,  but  this  is  not  so.  We 
have  now  in  our  possession  a  cheap,  but  well  made  microscope, 
the  rack  and  pinion  of  which  is  so  delicate,  that  with  it  we  can 
focus  an  objective  of  an  eighth  of  an  inch  focal  distance  with 
sufficient  accuracy  for  all  ordinary  purposes. 

For  ordinary  purposes,  especially  the  work  of  the  physician 
and  medical  student,  the  coarse  adjustment  may  be  more  easily 
dispensed  with  than  the  fine  one,  but  at  the  same  time  it  must 
be  remembered  that  any  mode  of  adjustment  in  which  the  body 
is  liable  to  turn  round,  is  incompatible  with  the  use  of  many 
important  pieces  of  apparatus.  Thus,  for  example,  any  turn- 
ing of  the  body  interferes  with  the  use  of  the  double  nose- 
piece,  the  polariscope  in  its  higher  applications,  Prof.  Smith's 
opaque  illuminator,  etc.  A  rack  and  pinion,  or  its  equivalent, 
should,  therefore,  always  be  chosen,  especially  as  it  does  not 
add  more  than  five  or  six  dollars  to  the  cost  of  the  instrument. 


OF  THE  MICROSCOPE.  119 

i 

Of  devices  for  fine  movements  the  name  is  legion.  An  old 
plan  is  to  place  the  object  upon  a  plate  attached  to  the  stage, 
and  move  it  towards  the  objective  by  means  of  a  fine  screw. 
This  is  a  cheap  and  convenient  method.  It  has  been  objected 
to  it  that  the  object  is  "tilted,"  as  it  were,  but  this  is  imper- 
ceptible in  practice,  and  causes  no  difficulty.  The  objection 
is.  that  when  the  object  moves,  many  of  the  finer  methods  of 
illumination  are  disturbed.  Another  common  plan  is  to  make 
the  nose-piece,  which  holds  the  objective,  movable.  This  alters 
the  length  of  the  body,  and  changes  the  magnifying  power 
every  time  a  change  is  made  in  the  focal  adjustment.  This 
change  is  too  slight  to  be  observable,  but  it  is  sufficient  to  inter- 
fere with  delicate  micrometric  measurements.* 

To  avoid  this  difficulty,  the  entire  body  and  its  attachments, 
including  the  coarse  movement,  are  carried  by  the  fine  adjust- 
ment. In  its  general  features,  this  plan  is  a  very  old  one,  and 
at  least  a  dozen  modifications  of  it  have  been  devised  by  dif- 
ferent makers.  As  usually  constructed,  the  body  is  raised  by 
means  of  a  lever,  the  long  arm  of  which  is  acted  upon  by  a 
delicate  screw.  A  strong  spring  is  arranged  to  lower  the  body, 
and  as  the  spring  maintains  a  continuous  action,  all  lost  motion 
is  prevented.  In  the  fine  motion  used  by  the  Bausch  &  Lomb 
Optical  Co.,  and  which  was  invented  by  Mr.  Gundlach,  the  body 
is  suspended  on  two  parallel  springs,  slides  on  carefully  planed 
ways,  and  is  moved  by  a  fine  screw. 

Objectives  of  large  aperture  are  so  sensitive  to  slight  varia- 
tions in  focal  adjustment,  that  it  is  difficult  to  get  a  fine  adjust- 
ment sufficiently  delicate.  Mr.  Gundlach  has  recently  at- 
tempted to  increase  this  delicacy  by  using  the  well-known  dif- 
ferential screw,  and  with  very  satisfactory  results. 

In  judging  of  the  quality  of  either  a  fine  or  coarse  adjustment, 
the  points  to  be  observed  are  the  delicacy  and  accuracy  with 
which  the  objective  may  be  moved  to  and  from  the  stage,  and 

*It  has  been  alleged  that  this  increase  or  decrease  of  magnifying  power 
is  more  apparent  with  the  higher  powers  than  with  the  lower  powers. 
Indeed,  it  has  been  said  that  with  high  powers  the  change  of  magnifying 
power  is  quite  perceptible.  This,  of  course,  is  mere  imagination,  as  any 
one  of  an  arithmetical  turn  of  mind  can  see.  Indeed,  the  facts  would 
seem  to  be  rather  the  other  way. 


120  SELECTION  AND  USE 

the  freedom  from  twist  or  apparent  displacement  of  the  object. 
In  many  microscopes,  when  a  high  power  is  used,  and  the 
body  is  moved  up  or  down  for  the  purpose  of  adjusting  the 
focus,  the  object  is  actually  thrown  out  of  the  field  of  view. 
Such  a  microscope  should  be  at  once  condemned. 

Whatever  be  the  nature  of  the  fine  or  coarse  adjustment,  see 
that  the  body  has  sufficient  range  of  motion  to  allow  of  the  use 
of  objectives  of  considerable  working  distance. 

Tlie  Diaphragm.— Nothing  tends  so  much  toobscureour 
view  of  the  finer  points  of  structure  in  any  object  as  to  have 
them  "drowned  "in  a  superabundance  of  light,  consequently 
in  order  to  regulate  the  amount  of  light  which  passes  through 
the  object,  a  diaphragm  is  employed.  As  ordinarily  con- 
structed, it  is  simply  a  metal  plate  placed  below  the  stage,  and 
pierced  with  holes  of  various  sizes,  which  may  be  brought  ex- 
actly under  the  field  of  view,  the  small  holes  allowing  but  a 
small  amount  of  light  to  pass,  while  the  large  ones  admit  a  full 
stream.  Considerable  difference  of  opinion  exists  amongst 
microscopisis  in  regard  to  the  proper  position  of  the  dia- 
phragm. Thus  Carpenter  says  (page  133)  that  unless  placed 
half  an  inch  below  the  object  it  is  comparatively  inoperative. 
Continental  histologists,  on  the  other  hand,  allege  that  it  is 
useless  unless  placed  close  up  under  the  object.  Microscopes 
constructed  according  to  both  these  plans  are  to  be  found  in 
market.  Where  the  microscope  is  furnished  with  a  sub-stage, 
the  distance  of  the  diaphragm  from  the  object  is  variable  at  will. 

It  is  obvious  that  when  the  diaphragm  is  placed  at  a  consid- 
erable distance  below  the  object,  the  illumination  is  purified, 
as  it  weiv,  from  all  cross  rays.  When  the  diaphragm  is  placed 
close  to  the  object-slide,  the  illuminated  field  is  contracted. 
The  action  in  this  case,  however,  is  somewhat  complex, 
owing  to  the  influence  of  the  slide  in  modifying  the  course  of 
the  rays. 

Several  very  ingenious  forms  of  Iris  or  graduating  dia- 
phragms have  been  devised,  by  which  the  size  of  the  hole  may 
be  changed  without  interrupting  the  observation.  They  are 
exceedingly  convenient,  and  present  advantages  which  more 
than  counterbalance  the  cost. 


OF  THE  MICROSCOPE.  121 

Objectives.— These  are  confessedly  the  most  important 
parts  connected  with  the  microscope;  they  therefore  deserve 
the  greatest  care  in  their  selection.  In  a  former  section,  we 
fully  explained  the  general  characteristics  of  the  different 
kinds  of  objectives  in  market,  and  detailed  the  best  methods  of 
of  testing  them.  A  careful  study  of  that  chapter  will,  we  hope, 
enable  the  beginner  to  avoid  a  glass  that  is  absolutely  bad, 
though  we  must  acknowledge  that  all  experienced  microscopists 
are  agreed  that  no  amount  of  mere  reading  will  enable  a 
novice  to  pronounce  a  correct  j  udgment  upon  the  quality  of  an 
objective,  unless  its  defects  should  be  very  glaring  indeed.  In 
this  place  we  shall  confine  ourselves  to  a  few  hints  in  re- 
gard to  those  features  which  adapt  objectives  not  only  to 
special  kinds  of  work,  bat  to  the  skill  of  different  classes  of 
workers.  For  it  is  an  undoubted  fact  that  objectives  which  in 
the  hands  of  skillful  microscopists,  and  on  certain  classes  of 
work,  would  give  extraordinary  results,  would  in;  other  hands, 
and  for  other  purposes,  prove  of  far  less  value  than  lenses  of 
what  is  commonly  considered  a  greatly  inferior  grade. 

We  do  not  here  propose  to  take  part  in  what  is  called  the 
"battle  of  the  object-glasses,"  such  a  discussion  being  out  of 
place  in  an  elementary  work  like  the  present,  but  we  think  few 
will  be  hardy  enough  to  deny  that  one  who  has  a  taste  for  such 
things,  but  has  neither  the  money  required  to  purchase  a  first 
class  glass,  nor  the  time  necessary  to  acquire  the  requisite  skill 
to  use  it,  had  better  work  with  a  cheap  French  triplet  than  not 
work  at  all.  Moreover,  it  is  astonishing  how  far  patience,  skill 
and  experience  will  go  to  make  up  for  a  deficient  instrument, 
while  at  the  same  time,  it  is  unfortunately  true  that  some  who 
possess  the  very  bast  glasses,  and  have  done  the  most  to 
throw  ridicule  upon  all  work  done  with  inferior  lenses,  have 
never  made  a  single  contribution  of  the  slightest  importance 
to  any  department  of  microscopical  science. 

In  a  former  chapter  we  discussed  at  length  the  different 
qualities  of  object-glasses,  and  showed  how  these  various  quali- 
ties might  exist  in  very  different  degrees  in  different  ob- 
jectives. It  is,  of  course,  obvious  that  the  extent  to 
which  any  one  quality  should  be  sought  in  a  particular 
glass,  must  depend  altogether  upon  the  kind  of  work  that  is 


122  SELECTION  AND     USE 

to  be  done.  To  those  who  are  addicted  to  what  Holmes  calls 
"fighting  objectives,"  resolution  will  be  the  quality  to  be 
desired;  others  will  prefer  penetration,  flatness  of  field,  etc. 
In  our  estimation,  for  tho  purposes  of  ordinary  scientific  work, 
we  would  assign  to  these  qualities  values  in  the  following 
order:  1.  Defining  power;  2.  Freedom  from  aberration  of 
form;  3-4.  Resolution  or  Penetration;  5.  Working  distance; 
6.  Achromatism;  7.  Flatness  of  field.  The  first  quality  that  should 
be  secured  in  every  lens  is  undoubtedly  defining  power,  and  this 
whether  its  angular  aperture  be  high  or  low.  Achromatism  we 
place  low  in  the  scale,  because  unless  so  marked  as  to  injure 
the  definition,  a  little  color  does  110  harm.  Flatness  of  field  we 
place  last,  because  it  will  be  found  that  perfect  flatness  of  field 
is  very  seldom  combined  with  first  rate  definition.  Indeed,  we 
have  heard  one  of  the  most  celebrated  makers  of  objectives 
assert  that  the  two  qualities  are  to  a  certain  extent  antagonistic. 
In  giving  advice  in  regard  to  the  selection  of  an  objective, 
one  of  the  points  concerning  which  it  is  most  difficult  to  arrive 
jit  a  decision,  is  that  of  angular  aperture.  Fortunately,  however, 
experienced  microscopists  may  safely  be  left  to  decide  this  ques- 
tion for  themselves,  and  since  those  who  have  had  no  experience 
will  find  it  difficult  to  use  objectives  of  very  wide  aperture,  it  will 
certainly  be  prudent  for  them  to  choose  those  of  moderate  angle. 
Objectives  of  very  high  angle  are  worthless,  unless  the  illumina- 
tion is  well  managed,  and  the  adjustment  for  thickness  of  cover 
properly  regulated.  Oa  the  other  hand,  a  good  non-adjusting 
lens  will  give  very  fair  results,  with  but  a  moderate  amount  of 
skill  on  the  part  of  the  user.  Almost  all  our  best  makers  now 
produce  objectives  of  moderate  angle,  which  do  not  adjust  for 
thickness  of  cover,  but  which  have  considerable  resolving 
power.  We  have  now  before  us  a  one-fifth  which  cost i  but  fif- 
teen dollars,  and  which  will  easily  resolve  the  Pleurosigma  An- 
gulatum  by  central  light.  An  important  point  for  consideration 
will,  of  course,  be,  whether  or  not  the  glass  is  intended  for 
original  work,  or  merely  for  the  study  or  exami  nation  of  well 
known  objects.  The  work  of  the  physician  is  chiefly  amongst 
well  known  objects,  and  may  be  very  satisfactorily  accomplished 
by  means  of  good  non-adjusting  objectives,  a  great  point  in 
favor  of  such  glasses  being  that  work  may  be  done  with  them 


OF    THE    MICROSCOPE.  123 

more  rapidly  than  with  glasses  that  require  greater  care  and 
skill.  The  same  is  true  of  the  elementary  studies  of  the  bot- 
anist and  histologist,  carried  on  under  the  guidance  of  a  com- 
petent teacher.  And  as  in  all  such  cases  it  is  easy  to  find  out 
the  special  thickness  of  glass  for  which  the  object-glass  has 
been  corrected,  and  to  provide  a  supply  of  the  proper  thickness, 
the  absence  of  a  means  of  adjustment  for  cover  thickness  is  not 
very  important.  But  for  all  the  higher  class  of  studies,  good 
glasses,  with  well-made  adjustments  for  thickness  of  cover,  are 
indispensable.* 

Objectives  of  very  low  angular  aperture,  are,  however,  to  be 
carefully  avoided.  There  is  a  want  of  light,  and  an  indistinct- 
ness which  renders  them  worthless.  It  is  generally  said  that 
the  superiority  of  large  angles  is  most  marked  in  the  objectives 
of  high  power,  and  that  for  low  powers  the  common  objectives 
do  very  well.  la  our  judgment,  however,  the  superiority  of 
the  low  powers  is  quite  as  marked  as  that  of  the  higher  ones, 
and  much  more  available  to  the  beginner.  It  is  true  that  the 
superiority  of  a  well  made  one-sixth  of  high  angle,  over  any 
triplet  of  whatever  focal  length,  is  immeasurable,  but  at  the 
same  time  it  is  equally  true  that  the  view  of  an  opaque  object 
seen  through  an  inch-and-a-half  objective,  carefully  corrected,  is 
as  much  superior  to  the  sam3  as  seen  through  a  common  triplet, 
as  it  is  possible  to  imagine.  We  have  now  before  us  a  speci- 
men of  bone  of  very  open  structure,  mounted  as  an  opaque 
object.  Seen  through  a  first  class  inch-and-a-half  objective,  it 
presents  almost  a  stereoscopic  appearance,  and  the  entire  struc- 
ture is  easily  made  out.  The  view  afforded  by  a  very  fair  French 
triplet  (No.  0)  is  so  markedly  inferior,  that  any  person  who 
should  see  the  two  would  never  again  use  a  cheap  objective,  if 
he  could  afford  to  get  a  good  one.  Moreover,  the  objection 
which  we  have  just  urged  against  objectives  of  high  power,  and 

*It  is  not  long  since  a  professional  maker  of  microscopes,  and  one  who 
seema  to  stand  high  in  the  favor  of  tha  medical  profession,  tried  to  per" 
suade  the  author  that  the  covering-glass  exercised  no  influence  on  the 
action  of  the  objective,  and  that  a  non-adjusting  glass  could  be  made 
with  as  great  a  resolving  power,  as  one  constructed  so  as  to  adjust  for 
different  thicknesses  of  covering  glass  1  To  such  men,  a  famous  micro- 
scopist  used  to  apply  the  term  "  shopticians,"  and  they  deserve  it. 


124  SELECTION   AND    USE 

wide  angle,  viz. ,  that  tliej  are  difficult  to  use  by  novices,  does 
not  hold  iti  the  case  of  low  powers.  A  good  inch,  of  compara- 
tively high  angle,  is  more  easily  used  than  a  poor  triplet. 

A  question  which  has  considerably  occupied  the  attention  of 
inicroscopists,  is  the  value  of  objectives  of  high  power,  and  their 
efficiency  as  compared  with  those  of  lower  denominations. 
That  in  many  cases  considerable  amplification  or  magnifying 
power  is  absolutely  necessary,  admits  of  no  doubt;  bat  the  ques- 
tion to  be  settled  is:  suppose  that  we  wish  a  power  of  2, 000  diame- 
ters, would  it  be  better  to  get  this  by  means  of  a  tenth  of  an  inch 
objective,  magnifying  100  times,  and  a  half  inch  eye-piece  mag- 
fying  20  times,  or  by  a  twentieth  of  an  inch  objective  magnify- 
ing 200  times,  and  an  inch  eye-piece  magnifying  ten  times  ? 

It  is  not  very  miny  years  ago  since  one  of  our  ablest  Ameri- 
can objective  makers  held  that  a  lens  of  a  quarter  of  an  inch 
focus  might  be  made  to  do  anything  that  a  lens  of  any  power 
could  be  made  to  do,  and  the  ground  of  this  opinion  was  that 
the  individual  lenses  of  objectives  as  low  as  a  fourth,  could  be 
made  so  much  more  perfect  than  the  smaller  lenses  of  higher 
powers,  that  this  perfection  more  than  cjunterbalanccd  the 
greater  magnifying  power  of  the  objective  of  shorter  focus. 
The  reasoning  here  seems  sound  and  obvious,  but  it  has  been 
found  in  practice  that  for  everything  except  resolution,  the  limit 
to  which  the  power  of  objectives  may  be  carried,  is  far  beyond 
a  fourth.  For  resolution  it  has,  we  believe,  been  found  that  a 
well  made  tenth  is  capable  of  doing  anything  that  any  lens  can 
do;  for  other  kinds  of  work  sixteenths  and  twenty-fifths,  and 
even  fiftieths  and  eightieths  have  been  declared  to  possess 
advantages  that  are  obvious.  This,  however,  is  one  of  those 
points  upon  which  authorities  differ;  Beale,  for  example,  favors 
high  powers;  Carpenter  and  Frey  seem  inclined  to  think  that 
very  high  powers  show  nothing  that  cannot  be  seen  by  means 
of  objectives  of  greater  focal  length. 

French  objectives  of  the  numbers  1,  2,  3  and  4,  if  carefully 
selected,  are  capable  of  doing  really  serviceable  work.  A  few 
years  ago,  some  of  the  best  known  makers  of  American  micro- 
scopes used  nothing  else,  even  in  microscopes  costing  $150,  but 
this  course  we  can  scarcely  regard  as  judicious,  for  whenever 
the  microscopist  is  prepared  to  expend  $75  or  more  for  a  micro- 


OF    THE    MICROSCOPE.  125 

scope,  a  large  part  of  this  sum  should  be  laid  out  iu  the  pur- 
chase of  objectives  of  the  better  class,  the  one-inch  and  one- 
fourth,  or  the  three-fourths  and  one-fifth  being  those  that  are 
usually  selected  by  beginners. 

French  triplets  are,  however,  going  rapidly  out  of  use,  from 
the  fortunate  circumstance  that  objectives  of  low  price  and 
excellent  quality  are  now  produced  by  several  makers  of  repute. 
It  is  well,  however,  for  the  reader  to  be  on  his  guard  against 
a  fraud  which  has  been  but  too  common  of  late  years.  Some 
so-called  opticians  go  so  far  as  to  add  a  little  brass- work  and 
engraving,  and  sell  these  French  triplets  as  objectives  of  Ameri- 
can make.  We  do  not  here  refer  to  the  mere  operation  of  attach- 
ing the  objective  to  an  adapter,  and  fitting  it  in  a  brass  box,  for 
this  adds  greatly  to  the  convenience  with  which  such  minute 
objectives  may  be  handled  and  preserved,  but  to  a  sort  of 
" making  over,"  by  which  they  are  completely  disguised  and 
made  to  resemble  the  objectives  of  English  and  American 
makers.  It  is  hardly  necessary  to  characterize  such  a  pro- 
ceeding. 

Eye-Pieces.— The  eye-piece  that  is  at  present  almost  uni- 
versally used  is  the  Huyghenian,  which,  when  well  made,  gives 
vqry  excellent  results.  In  the  use  of  low  powers,  wheie  a  very 
flat  and  large  field  is  desirable,  the  Huyghenian  eye-piece  fails, 
and  the  same  is  also  true  in  regard  to  very  high  magnifying 
powers,  where  the  enlargement  is  obtained  in  a  great  measure 
by  means  of  the  eye-piece.  The  extent  to  which  the  definition 
of  really  good  objectives  is  deteriorated  by  the  use  of  eye-pieces 
of  great  magnifying  power,  and  the  loss  of  light  which  they 
occasion,  render  them  practically  useless.  For  high  powers, 
the  solid  eye- pieces  of  Mr.  Tolles  are  vastly  superior,  while  for 
low  powers,  where  a  large  flat  field  is  desired,  Kelner's  ortho- 
scopic  eye-piece  presents  important  advantages. 
.  Mr.  Gundlach  has  recently  brought  out  a  new  eye-piece, 
which  he  has  named  the  periscopic,  and  for  which  a  large  field 
and  excellent  definition  are  claimed.  They  are  much  more  ex- 
pensive than  the  Huyghenian.  We  have  not  had  an  oppor- 
tunity of  examining  them  carefully. 

In  determining  the  quality  of  an  eye-piece,  attention  is  to  be 


126  SELECTION   ANB    ttSfi 

paid  not  only  to  its  general  excellence,  bat  to  its  adaptability 
to  the  objectives  that  are  to  be  used  with  it.  In  the  higher 
departments  of  microscopy,  the  latter  is  a  most  important  point, 
but  one  which  is  too  frequently  neglected.  It  does  not,  how- 
ever, come  within  the  scope  assigned  to  the  present  work,  and 
we,  therefore,  content  ourselves  with  a  few  general  hints. 

The  lenses  composing  the  eye-piece,  should  be  of  homogeneous 
glass— that  is,  free  from  air-bubbles,  specks  and  striae,  and  the 
surfaces  should  be  well  polished.  These  points  require  atten- 
tion, because  we  have  in  our  possession  a  microscope  in  which — 
though  it  cost  enough  money  to  be  free  from  such  defects — 
they  are  glaringly  apparent.  On  looking  through  the  eye- 
piece at  a  strongly  and  evenly  illuminated  surface,  the  entire 
field  of  view— that  is,  the  whole  of  the  bright  circle  that  is 
seen,  should  have  the  light  evenly  diffused  over  its  surface, 
and  the  edges  or  border  of  this  circle  should  be  sharp  and 
black. 

Eye-pieces  intended  for  first-class  objectives  should  give  a 
large  field  of  view;  but  on  the  other  hand,  if  French  objectives 
be  used,  the  field  of  view  should  be  small,  otherwisa  the  defi- 
nition will  be  poor.  This  is  a  point  that  is  frequently  over- 
looked, and  we  have  seen  very  fair  object-glasses  condemned 
as  worthless  when  used  with  a  stand  and  eye-piece  intended 
for  objectives  of  an  entirely  different  class.  It  is  an  easy  thing 
to  contract  the  field  of  view,  by  means  of  a  round  piece  of  thin 
sheet  metal,  having  a  hole  of  proper  size  in  the  centre.  As  pre- 
viously explained,  such  a  piece  of  metal  is  called  a  diaphragm, 
and  should  always  be  well  blackened. 

The  magnifying  power  of  every  microscope  depends  upon 
three  things:  The  focal  length  of  the  objective,  the  length  of 
the  body,  and  the  eye-piece.  Most  microscopes  are,  therefore, 
furnished  with  several  eye-pieces,  whereby  the  magnifying 
power  may  be  varied.  There  is,  however,  a  limit  to  the  extent 
to  which  this  may  be  done.  The  image  obtained  by  very  deep 
eye-pieces,  as  they  are  called,  is  rarely  satisfactory. 

The  different  eye-pieces  are  generally  denoted  by  letters — A, 
B,  C,  D,  etc.  A  being  the  lowest,  and  B,  C,  D,  et£.,  successively 
higher.  Some  makers  use  numbers — 1,  2,  3,  4,  etc.  These 
letters  and  number,  are,  however,  entirely  arbitary,  in  this  point 


Off  TH£  MICfcOSCOfE.  127 

resembling  the  numbers  assigned  to  objectives  by  continental 
makers.  A  great  improvement  upon  tins  arbitrary  and  uncer- 
tain system  would  be  to  assign  to  each  eye-piece  its  proper 
power  expressed  in  inches.  Thus,  an  eye-piece  magnifying  the 
same  as  a  simple  lens  of  two  inches  focus,  should'be  called  the 
two-inch  eye-piece. 

And  here  let  us  call  attention  to  the  terms  deep  and  shallow,  as 
applied  to  eye-pieces.  By  all  authors  of  repute,  a  deep  eye-piece 
is  one  of  great  magnifying  power,  while  a  shallow  eye-piece  it  the 
reverse.  See  the  Micrographic  Dictionary,  and  the  works  of  Car- 
penter, Beale,  Lardner,  Frey,  etc.,  etc.  It  is,  therefore,  singu- 
lar that  Dr.  Lankester,  in  his  popular  little,  work,  "  Half-Hours 
with  the  Microscope,"  should  have  committed  the  mistake  of 
giving  definitions  exactly  the  opposite,  upon  the  ground 
that  eye-pieces  of  great  magnifying  power  are  always  short, 
while  low  eye-pieces  are  always  long.  It  is  evident,  however, 
that  the  terms  are  liable  to  give  rise  to  confusion,  and  we  pre- 
fer the  words  high  and  low — the  meaning  of  which  is  so  ob- 
vious as  to  require  no  explanation,  as  every  body  knows  what 
high  magnifying  power  is 

While  clearness  of  definition  and  resolving  power  are  the 
most  important  qualities  of  every  good  microscope,  magnifying 
power  is  also  of  considerable  consequence,  as  explained  in  a 
former  section.  Therefore,  every  good  microscope  should  be 
provided  with  at  least  one  eye-piece  of  considerable  power.  It' 
often  happens  that  with  the  objectives  and  eye-pieces  at  hand, 
the  amplification,  as  it  is  called,  or,  in  other  words,  the  extent  to 
which  the  object  is  magnified,  is  nol  sufficiently  great  to  enable 
us  to  make  out  its  structure,  while  the  objective  has  not  by 
any  means  reached  the  limit  of  its  defining  power.  In  this 
case  a  high  power  eye-piece,  which  costs  comparatively  little, 
will  greatly  extend  our  power  of  successful  examination. 

ACCESSORY  APPARATUS. 

Every  microscope  should  be  accompanied  with  certain  pieces 
of  accessory  apparatus,  which  are  necessary  for  the  convenient 
and  thorough  examination  of  objects,  but  which  do  not  form 
part  of  the  instrument  itself.  Some  of  these  are  intended  for 


128 


SELECTION    AND 


the  better  illumination  of  the  ob- 
ject, and  will  be  described  in  the 
section  on  "Light;"  others  are  used 
for  the  procuring  and  preparation 
of  objects,  and  will  be  described  in 
the  section  devoted  to  tbet  subject. 
The  following  are  employed  chiefly 
for  holding  and  presenting  objects 
that  have  not  been  "  mounted:" 


Stage  Forceps. —This  little  in- 
strument accompanies  the  oldest 
microscopes.  It  consists  of  a  pair 
of  very  delicate  forceps,  such  as 
those  attached  to  the  forceps-car-, 
rier  in  Fig.  27,  which  close  by  the 
spring  of  the  jaws,  and  hold  any  ob- 
ject that  may  be  placed  in  their 
grasp.  They  are  opened  by  pressing 
on  the  pins  which  are  seen  at  the 
sides.  They  are  in  general  fastened 
to  tne  microscope  by  being  stuck 
into  a  hole  in  the  stage,  and  the 
object  may  not  only  be  moved  back- 
ward and  forward,  but  it  can  be 
turned  round.  The  better  class  of 
'forceps  carry  a  small  /brass  tube 
(shown  in  Fig.  27)  which  is  filled 
with  cork,  and  which  serves  to  re-' 
ceive  pins,  etc.,  for  holding  insects, 
and  other  objects. 

Foi  ceps-Carrier.  —  However 
we-1  made  the  forceps  may  be,  it  is 
almost  impossible  to  slide,  with  suf- 
ficient delicacy,  the  rod  through  the 
tube  that  holds  it.  Consequently, 
it  is  exceedingly  difficult  to  bring 
into  the  field  of  view,  the  exact  part 
of  the  object,  that  we  may  wish  to 


Off  THE  MICROSCOPE.  129 

examine.  To  avoid  this  difficulty,  the  author,  instead  of  insert- 
ing the  pin  of  the  forceps  in  the  stage,  provides  a  special  for- 
ceps-carrier like  that  shown  in  Fig  27.  This  consists  simply  of 
a  metal  plate,  the  size  of  an  ordinary  slide,  and  having  a  hole  in 
one  end  to  receive  the  pin  of  the  forceps.  A  large  hole  is 
pierced  through  the  centre,  to  allow  the  passage  of  light  from 
the  mirror  when  that  is  needed.  This  plate  is  placed  on  the 
stage  like  a  common  slide,  and  it  can  be  moved  with  as  great 
delicacy  as  any  ordinary  object.  The  mode  of  using  it  is  too 
obvious  to  require  further  explanation.  We  have  found  it  ex- 
ceedingly convenient. 

Object-Holder.— The  importance  of  being  able  to  present 
an  object  to  the  light  in  all  directions  is  well-known  to  every 
microscopist.  Many  years  ago  we  devised  an  object-holder  for 
effecting  this,  the  construction  of  which  is  very  simple  and  in- 
expensive. It  consists  of  a  slip  of  metal,  the  size  of  an  or 
dinary  slide — three  inches  by  one — having  a  hole  in  the  centre 


Fig.  28. — OBJECT-HOLDER. 

and  a  short  pillar  rising  from  one  end,  as  shown  in  the  en- 
graving,  Fig.  28,  which  gives  a  sectional  elevation  of  the 
instrument.  Through  this  pillar  runs  a  wire,  carrying  at  one 
end  a  milled  head  by  which  it  may  be  turned,  and  at  the  other 
a  ring  which  holds  a  perforated  block.  This  perforated  block 
has  a  milled  collar  on  the  lower  end,  so  that  it  can  be  readily 
turned  in  the  ring  that  carries  it.  The  hole  passing  through 
the  block  is  just  the  size  of  a  stout  pin,  so  that  a  disk  of  card 
or  leather,  with  a  pin  through  it,  will  be  held  steadily  when 
the  pin  is  inserted  in  the  hole.  The  object  to  be  examined  is 
attached  to  the  surface  of  the  card,  by  means  of  balsam  or  mu- 
cilage, and  it  is  obvious  that  by  the  combined  rotations  that 
may  be  produced  by  the  two  milled  heads  mentioned,  it  may 
be  exposed  to  the  action  of  the  light  in  any  desired  manner. 


130  SELECTION  AND    USB 

The  changes  which  are  produced  in  some  objects  when  the 
light  is  made  to  full  on  then  in  different  directions  are  very 
marked.  Thus,  for  example,  the  mineral  known  as  specular 
iron  ore,  when  illuminated  by  light  falling  on  it  in  one  direc- 
tion, is  brilliant  in  the  extreme,  while  when  the  light  falls  in 
other  directions  it  is  dead  and  lustreless.  Arid  as  it  is  not 
always  convenient  to  change  the  position  of  the  lamp,  it  is  a 
great  advantage  to  be  able  to  tarn  the  object  round.  The  sim- 
ple contrivance  just  described  enables  us  to  do  this  perfectly. 

A  more  perfect  arrangement,  intended  for  the  same  purpose, 
has  been  devised  by  Mr.  Beck,  of  London.  Mr.  Beck's  is, 
however,  more  expensive  than  ours. 

Plain  Slides.— The  common  plain  slides  serve  very  well 
for  examining  ordinary  deposits  in  liquids.  This  is  particu- 
larly the  case  where  inanimate  objects,  vegetables  and  minerals 
are  to  be  examined.  Active  animals  require  some  contrivance 
for  keeping  them  still. 

Tlie  Concave  Slide,  as  it  is  called,  is  simply  a  thick 
slide  with  a  cup-like  hoilow  ground  in  the  centre.  Such  slides 
are  cheap,  and  very  convenient.  A  drop  of  water  placed  in  one 
of  these  concaves,  and  covered  with  a  thin  glass,  may  be  exam- 
ined easily  and  thoroughly  with  moderate  power.  It  is 
sometimes  desirable  to  employ  a  cell  with  a  perfectly  flat  bot- 
tom of  very  thin  glass.  Such  cells  may  be  easily  and  conve- 
niently made  out  of  a  slide  of  metal,  or  preferably  of  vulcanite, 
through  which  a  hole  the  size  of  the  proposed  cell  has  been 
pierced.  A  piece  of  thin  glass  may  then  be  cemented  to  the 
under  side  of  the  slide,  so  as  to  form  a  water-tight  cup.  The 
hole  in  our  slides  is  round,  and  has,  on  the  under  side,  a  seat 
or  rebate,  a  little  larger  than  the  hole  itself.  In  this  rebate  a 
round  glass  cover  fits,  so  as  to  leave  the  under  side  of  the  slide 
perfectly  smooth.  Such  cells  are  very  convenient,  as  they  are 
easily  cleaned,  and  are  not  difficult  to  repair  when  the  thin 
glass  gets  broken.  The  liquid  is  also  easily  covered  by  means 
of  a  thin  glass  cover,  and  when  full,  considerable  inclination 
may  be  given  to  the  slide  before  the  liquid  shows  a  tendency  to 
run  out.  Various  other  devices  of  a  simple  kind  may  be  con- 
trived by  the  microscopist  for  similar  purposes. 


OF    THE    MICKOSCOPE.  131 

Watch-Glasses.— Dr.  Beale  recommends  small  flat  watch- 
glasses  for  holding  liquids  that  are  to  be  examined,  and  we 
Lave  found  them  very  excellent.  The  best  kind  for  this  pur- 
pose are  those  known  as  lunette  glasses,  which  are  nearly  flat  on 
the  bottom.  They  are  awkward  things  to  manipulate,  how- 
ever, unless  some  means  is  provided  for  holding  them  steady, 
and  moving  them  about  on  the  stage.  We  use  for  this  purpose 
a  strip  of  wood,  three  inches  long,  and  so  wide  that  we  can 
easily  bore  in  it  a  hole,  about  one-eighth  of  an  inch  less  in 
diameter  than  the  watch-glass,  of  which  the  smallest  size  should 
be  chosen.  The  thickness  of  the  strip  should  be  sach  that 
when  laid  on  any  flat  surface,  the  watch-glass  will  not  come  in 
contact  with  it.  Glasses  held  in  this  way  are  very  convenient. 


Fig.  29.— WATCH-GLASS  HOLDER. 

Watch-glasses  are  very  convenient  for  examining  a  "dip  "  from 
a  pond  or  stream,  but  for  this  purpose  they  require  a  holder. 
The  little  instrument  shown  in  the  cut  is  formed  of  three  pieces 
of  bright  tin,  which  are  hinged  together.  In  the  upper  piece 
is  cut  a  hole  just  large  enough  to  receive  a  watch-glass.  A  ring 


132  SELECTION  AND   USE 

of  metal  of  proper  height  surrounds  this  hole,  and  forms  a 
perfect  protection  to  the  glass  when  the  instrument  is  carried 
in  the  pocket.  The  lower  slip  of  tin  may  be  adjusted  to  any 
angle,  and  by  turning  towards  the  proper  direction,  the  light 
of  any  bright  cloud  may  be  reflected  up  through  the  liquid. 
All  the  joints  are  made  stiff  enough  to  remain  in  position  when 
once  adjusted.  A  watch-glass  arranged  in  this  manner  holds  a 
liberal  supply  of  liquid,  so  that  an  entire  "dip"  may  be 
readily  examined  at  once.  We  have  found  this  little  contrivance 
far  superior  to  more  expensive  arrangements.  It  packs  into 
small  compass,  and  is  safely  carried. 

Animalcule  Cage. — This  forms  a  very  excellent  means 
for  holding  animalcules  that  are  too  active  to  allow  of  observa- 
tion on  slides,  or  in  watch-glasses.     A  good  idea  of  its  con- 
struction may  be  ob- 

be  seen  to  consist  of  a 

by  one  inches,  to  the 
Fig.  30.— ANIMALCULE  CAGE.  centre  of  which  is 

fixed  a  short  tube.  In 

the  upper  end  of  this  tube  is  fastened  a  beveled  piece  of  glass, 
and  a  second  tube  fits  over  the  first,  and  has  a  thin  glass  cover 
secured  in  its  upper  end.  The  animalcule  is  securely  held 
between  the  two  pieces  of  glass,  and  the  lower  glass  being 
beveled  on  the  edge,  a  drop  of  liquid  placed  on  it  is  held  be- 
tween the  two  glasses  by  capillary  attraction,  and  cannot  spread 
over  the  inside  of  the  cage.  This  point  is  generally  neglected 
in  the  cheaper  forms  of  the  cage,  in  which  the  lower  glass 
is  simply  a  plain  disc  burnished  into  the  upper  end  of  the 
inner  tube.  The  consequence  is  that  when  the  two  glasses  are 
brought  together  the  liquid  flows  over  the  entire  inside  of  the 
cage,  and  the  objects  are  liable  to  be  floated  out  and  lost.  As 
it  is  important  that  the  distance  of  the  two  glasses  from  each 
other  should  be  easily  and  accurately  regulated,  the  outer  tube 
should  be  slit,  so  as  to  make  it  springy.  In  this  way  it  may  be 
made  to  move  with  a  soft  and  equable  motion. 


OP    THE    MICROSCOPE. 


133 


The  Zoophyte  Trough.  —This  little  piece  of  apparatus 
is  almost  indispensable  to  those  who  desire  to  watch  the  growth 
and  development  of  the  larger  animalcules  and  small  aquatic 

plants.  Several  forms  are  in 
common  use,  the  most  com- 
plete being  that  shown  in  Fig. 
31.  The  trough  itself  is  sim- 
ply a  glass  tank,  to  which  is 
fitted  a  slip  of  thin  plate  glass 
that  acts  as  a  division,  and 
enables  the  observer  to  keep 
the  objects  close  up  to  the 
front  plate.  The  distance  of 
the  dividing  plate  from  the 
front  plate  is  regulated  by  an 
ivory  wedge,  and  the  dividing 
plate  is  kept  firmly  up  to  its  place  by  means  of  a  spring.  This 
contrivance  enables  us  to  regulate  the  thickness  or  width  of  the 
tank,  so  that  the  interior  of  the  vessel  may  be  made  so  large 
that  it  can  be  easily  cleaned. 

form    of    the    Zoophyte    trough 
consists  of  a  simple  glass  box, 


Fig.  31.— ZOOPHYTE  TROUGH. 


A  smaller  and  simpler 
is  shown  in  Fig.  32.  It 
open  at  the  top.  The  back 
of  the  box  is  formed  of  a 
stoutish  piece  of  plate  glass, 
to  which  is  cemented  three 
glass  strips,  forming  the 
bottom  and  ends.  The  front 
is  formed  of  glass  as  thin  as 
is  compatible  with  durabil- 
ity, and  is  also  cemented  to 
the  end  pieces.  The  width 
of  the  trough  from  front  to 

back  is  generally  from  an  eighth  to  a  quarter  of  an  inch.  When 
the  trough  is  filled  with  water,  and  living  animals  are  placed  in 
it,  their  changes  and  movements  may  be  very  readily  watched. 

Small  troughs,  such  as  that  just  described,  are  not  difficult 
to  make,  though  the  very  low  price  at  which  they  are  sold  (60 
cents  to  $1.00)  renders  it  scarcely  worth  the  while  of  ordinary 


Fig.  32. — ZOOPHYTE  TROUGH. 


134  SELECTION    AND    USE 

microscopists  to  construct  them  for  themselves.  Where  this  is 
desirable,  however,  the  best  method  of  making  them  is  as 
follows  :  Select  a  piece  of  plate  glass,  of  the  thickness  of  an 
ordinary  slide,  and  cut  it  about  three  inches  by  one  and  a  quar- 
ter. Then  select  another  piece  of  glass,  as  thick  as  the  trough 
is  to  be  deep  (from  front  to  back),  and  cut  it  to  the  size  of  the 
outside  of  the  trough.  From  the  bottom  of  this  piece  of  glass 
cut  a  strip  a  quarter  of  an  inch  wide,  and  from  the  sides  also 
cut  strips  of  the  same  width.  The  centre  piece  may  now  be 
thrown  aside,  and  the  ends  of  the  bottom  strip  will  make  a 
tight  joint  with  the  side  strips.  The  three  strips  should  then  be 
cemented  to  the  large  plate,  and  over  them  should  be  cemented 
a  piece  of  the  thin  glass  used  for  covers.  The  strongest  cement 
is  marine  glue,  but  it  is  somewhat  difficult  to  use  by  those 
who  have  had  no  experience.  Prof.  Starr,  who  is  Avell  known 
for  his  success  in  keeping  and  exhibiting  living  microscopic 
objects,  uses  old  Canada  balsam,  and  we  have  seen  a  large 
variety  of  microscopic  animals  and  vegetables  which  had  been 
kept  for  months  in  a  healthy  condition  in  such  troughs  or 


Bottrell's  Zoophyte  Trough.— A  serious  objection 
to  the  troughs  which  we  have  just  described  is  the  difficulty  of 
cleaning  them,  and  of  repairing  them  when  broken.  To  avoid 
these  difficulties,  Mr.  Bottrell  has  devised  the  little  piece  of 


Fig.   33.— ZOOPHYTE  TROUGH. 

apparatus  shown  in  Fig.  33.  This  trough  consists  of  two  glass 
plates,  which  are  separated  by  a  semi-ring  of  vulcanized  india- 
rubber,  against  which  they  are  squeezed  so  firmly  as  to  be 
watertight,  by  means  of  two  vulcanite  plates,  held  together  by 
screws  with  milled  nuts.  The  front  plate  may  be  made  of 
thick  or  thin  glass,  as  desired,  while  the  depth  of  fluid  em- 


OF   THE   MICROSCOPE.  135 

ployed  may  be  regulated  by  the  thickness  or  number  of  semi- 
rings of  rubber  placed  between  the  glass  plates. 

By  means  of  the  three  screws  with  milled  nuts,  the  trough 
may  be  taken  to  pieces  in  a  few  seconds,  and  as  easily  put  to- 
gether. In  this  way  it  is  easy  to  clean  the  inside  of  the  glass 
plates,  to  replace  broken  ones,  etc.,  etc. 

The  Weber  Slide.— This  ingenious  device  consists  of  a 
common  slide,  rather  thicker  than  usual,  m  the  centre  of  which 
an  annular  groove  has  been  ground,  as  shown  in  Fig.  31.  The 
central  part  of  the  cell  is  left  nearly  the  full  original  height  of 
the  glass — just  enough  being  taken  off  to  allow  of  a  thin  layer 
of  liquid  between  it  and  the  cover-glass.  Any  liquid  contain- 
ing minute  forms  of  animal  or  vegetable  life  having  been  laid 
on  the  top  of  the  central  elevation,  will  be  held  there  by  capil- 
lary attraction  as  soon  as  the  cover  is  laid  on,  and  if  the  latter 
be  cemented  round  the  edges,  an  air-tight  aquarium  on  a 
minute  scale  is  formed,  in  which,  if  the  proper  balance  of 
animal  and  vegetable  life  is  present,  the  objects  may  be  kept  in 
good  condition  for  a  long  time.  We  have  kept  one  of  these 
slides  for  weeks  with  desmids,  diatoms,  and  minute  forms  of 
animal  life,  all  in  good  condition.  The  proper  cement  for  at- 
taching the  cover  to  these  slides  is  beeswax  softened  with  oil. 


Fig.  34. — THE  WEBEE  SLIDE. 

The  "Weber  slide  is  a  very  handy  and  convenient  piece  of  ap- 
paratus, but  it  is  difficult  to  obtain.  We  have,  therefore,  used 
the  following  form,  which  anyone  can  make  for  himself. 

The  Cell  Trough.— The  simplest  trough  in  which  living 
objects  may  be  kept  for  some  time,  is  constructed  out  of  an 
ordinary  cell  and  thin  cover,  as  shown  in  Fig.  35.  I*  we 
have  a  liquid  containing  some  very  minute  objects  which  we 
wish  to  keep  for  some  time  in  a  condition  suitable  for  examina- 
tion, we  place  a  drop  on  the  centre  of  a  thin  cover-glass,  which 
must  be  larger  than  the  cell.  A  very  small  cover  is  then  placed 


136  SELECTION  AND   USE 

over  the  liquid,  for  the  purpose  of  forming  a  layer  of  equal 
depth  throughout,  and  if  there  should  be  any  danger  of  crush- 
ing the  objects,  a  few  fibres  of  hair,  silk,  cotton,  etc.,  will  keep 
the  two  thin  glasses  sufficiently  apart.  The  edge  of  the  cell 
having  been  lightly  smeared  with  a  soft  mixture  of  beeswax  and 


Fig.    35. — CELIrTROUGH. 

oil.  and  the  interior  corner  having  been  moistened  with  pure 
water  by  means  of  a  camel-hair  pencil,  the  inverted  cell  is 
placed  on  the  large  cover,  which  at  once  adheres,  and  thus  the 
cell  is  converted  into  a  veritable  Wardian  case.  Owing  to  the 
moist  atmosphere,  there  is  no  danger  of  the  liquid  under  exam- 
ination drying  up,  and  as  the  apparatus  is  very  cheap,  several 
may  be  fitted  up  and  used  for  watching  the  life  history  of  any 
particular  subject. 

The  Compressorium.— The  animalcule  cage,  shown  in 
Figure  30  is  open  to  the  objection  that  it  is  difficult  to 
adjust  the  pressure  with  sufficient  delicacy,  and  we  are  apt 
either  to  crush  the  animal  or  leave  it  too  free  in  its  movements. 
These  difficulties  may  be  avoided  by  the  use  of  a  well-made 
Compressorium,  of  which  there  are  several  kinds  in  use. 

This  little  instrument  also  serves  another  important  purpose. 
In  the  examination  of  certain  objects,  it  is  frequently  necessary 
to  flatten,  and  even  to  crush  them,  in  order  to  render  their 
structure  visible,  and  this  the  compressorium  enables  us  to  ac- 
complish. The  most  powerful  instrument  of  this  kind  consists 
of  a  metal  plate,  in  the  centre  of  which  is  fixed  the  disc  of  glass 
upon  which  the  object  is  laid.  A  second  disc  of  glass,  fastened 
in  a  ring  which  is  hung  at  the  end  of  a  lever,  by  means  of  two 
pivots,  is  pressed  against  the  first  by  means  of  a  screw,  which 
tilts  the  lever.  In  this  way  a  very  strong  pressure  may  be 
exerted,  while,  o'vying  to  the  free  movement  of  the  ring  on  the 
pivots,  the  plates  of  glass  always  remain  parallel  to  each  other. 

Where  very  great  pressure  is  not  required,  a  different  form 
of  the  instrument  may  be  used.  Instead  of  being  forced  down 
Tbv  a  scr§w  uncl  lever,  the  upper  glasg  clisc  is  fastened  tq  a  thin 


OF  THE  MICBOSCOFE.  137 

plate  of  metal,  which  may  be  raised  by  a  screw,  but  when  the 
screw  is  withdrawn  the  ''spring"  of  the  plate  carries  it  down 
and  gives  the  pressure. 

A  compressorium,  in  which  the  pressure  is  caused  by  the 
weight  of  the  upper  plate,  is  shown  in  Fig.  36.  There  are  two 
plates  of  metal,  each  with  a  hole  in  the  centre,  to  receive  the  glass 
discs.  At  one  end  the  upper  plate  has  two  pins,  which  fit  into 
two  holes  in  the  lower  plate,  and  serve  to  prevent  all  side 
movements.  A  screw  passes  through  the  other  end  of  the 
upper  plate,  and  serves  to  separate  the  two.  A  drop  of  water 
containing  an  animalcule  having  been  placed  on  the  thin  glass 
attached  to  the  lower  plate,  the  upper  plate,  with  the  screw 
projecting  sufficiently  from  the  under  side,  is  laid  on  it.  Then 
by  turning  the  screw,  we  can  bring  the  two  plates  together  to 
any  required  degree  of  nearness,  and  with  the  utmost  delicacy. 


Fig.   36.— THE  GRAVITY  COMPRESSORIUM. 

Any  minute  animal  may  thus  be  firmly  grasped,  without  crush- 
ing if, while  the  compressing  power  exerted  by  the  mere  weight 
of  the  metal  plate  is  in  almost  all  cases  sufficient,  even  for  the 
complete  flattening  out  of  small  worms,  etc.  Even  such  crea- 
tures as  the  larva  of  the  common  gnat  or  mosquito  may  be 
completely  crushed  by  the  weight  of  a  plate  less  than  the  eighth 
of  an  inch  thick;  and,  where  greater  force  is  required,  it  is  of 
course  easy  to  apply  the  pressure  of  the  finger.  In  the  latter 
case,  no  danger  of  exerting  too  great  a  pressure  need  be  in- 
curred, as  the  projecting  screw  prevents  all  that.  The  want  of 
parallelism  between  the  plates  does  not  prove  a  serious  objec- 
tion, as  it  is  so  very  slight  that  it  is  hardly  perceptible  in  tho 
short  distances  ordinarily  under  observation.  Where,  how- 
ever, it  is  desirable  to  avoid  this  defect,  screws  may  be  substi- 
tuted for  the  pins,  and  the  points  may  be  made  to  work  in 
holes  bored  half  through  the  lower  plate. 


138  SELECTION  AND    USE 

Where  animalcule  cages  are  not  accessible,  a  small  animal 
may  be  held  between  a  common  slide  and  a  thin  cover.  To 
prevent  crushing  it,  a  hair  or  even  a  thread  may  be  placed  be- 
tween the  cover  and  the  glass.  A  German  author  recommends 
the  use  of  fine  gauze  or  netting,  in  the  meshes  of  which  an 
animalcule  may  be  held  very  conveniently.  Acting  on  this 
idea,  we  took  a  thin  metal  plate,  and  bored  it  full  of  holes  of 
various  sizes.  An  animalcule  placed  in  one  of  these  holes  may 
be  kept  in  the  field  of  view  for  any  length  of  time,  and  exhibited 
to  those  who  desire  to  see  it,  but  it  cannot  be  kept  quiet  for 
scientific  examination.  We  like  a  piece  of  fine  wire-gauze, 
better  than  cotton  or  linen  netting. 

Growing  Slides,— Where  it  is  desirable  to  keep  the  same 
living  object  for  a  considerable  time,  so  as  to  watch  its  changes, 
it  is  necessary  to  use  what  is  called  a  growing  slide,  by  whiqh  it 
may  be  regularly  supplied  with  air  and  moisture.  A  large  mirn- 


FJg.   37.— GROWING   SLIDE. 

ber  of  complicated  devices  have  been  described  for  this  pur- 
pose, but  the  following  simple  contrivance  answers  the  end 
very  well;  we  have  used  it  for  years.  To  one  end  of  a  common 
slide  with  a  concave  centre,  cement  a  small  bottle,  as  shown  in 
the  figure.  This  is  easily  done  by  means  of  a  little  marine 
glue.  The  glue,  cut  in  small  pieces,  should  be  laid  on 
the  slide  at  the  point  where  the  bottle  is  to  be  attached; 
the  slide  is  then  to  be  -gradually  heated  until  the  glue  is 
softened,  when  the  bottle  is  laid  on  and  moved  back  and 
forth  until  it  has  been  thoroughly  imbedded  in  the  cement. 
The  bottle  is  filled  with  water  and  corked,  the  upper  side 
of  the  cork  having  two  notches  cut  in  it,  one  for  the  en- 
trance of  air,  and  the  other  for  the  passage  of  a  loose  cotton 


OF    THE   inCBOSCOPE.  139 

Oread.  The  object  is  placed  in  tlie  concavity,  covered  with  a 
piece  of  thin  glass,  and  the  end  of  the  thread  is  carried  tinder 
the  cover  by  means  of  a  small  notch  cut  in  the  slide  with  a 
file.  The  bottle  must  be  filled  with  very  pure  water,  otherwise 
the  salts,  etc. ,  contained  in  it,  become  concentrated  under  the 
tMn  cover,  owing  to  the  evaporation,  and  destroy  the  object. 

Frog  Plate. — The  circulation  of  the  blood  in  the  capillaries 
of  living  animals  may  be  observed  in  the  web  of  the  frog's  foot, 
the  tail  of  a  small  fish  or  water-lizard,  the  larvse  of  many  insects, 
the  ear  of  a  young  mouse  and  the  wing  of  the  bat.  The  tongue 
of  the  frog  is  also  a  favorite  subject  with  some,  and  dissections 
of  the  living  animal  have  also  been  made,  and  the  circulation  ob- 
served in  the  parts  thus  displayed.  Except,  however,  for  im- 
portant investigations,  we  h,ave  no  right  thus  to  inflict  torture 
and  destroy  life,  and,  moreover,  the  obvious  cruelty  of  the 
means  employed,  will  to  most  minds  destroy  nearly  all  the  pleas- 
ure arising  from  the  beauty  of  the  exhibition.  Fortunately  the 
circulation  of  the  blood  in  the  foot  of  the  frog  may  be  witnessed 
without  subjecting  the  animal  to  any  pain.  For  this  purpose 
the  web  of  the  hind  foot  is  spread  out  over  a  piece  of  glass, 
which  is  held  in  a  frog-plate,  as  it  is  called,  to  which  the  little 
animal  is  attached.  The  frog-plates  usually  sold,  however,  do 
not  lie  conveniently  on  the  stage  of  a  small  microscope;  they 
are  apt  to  tip  up,  and  there  is  no  means  of  attaching  them 
firmly  to  the  stage,  so  that  it  is  impossible  to  incline  the  mi- 
croscope. The  annexed  engravings  represent  a  frog-plate,  in 
which  these  difficulties  are  avoided.  As  seen  in  the  figure,  it  is 
of  the  usual  form,  and  has  a  large  opening,  into  which  is 
burnished  a  piece  of  thinnish  plate  glass  upon  which  the  web 
of  the  foot  is  laid.  Around  this  opening  is  bored  a  number  of 
small  holes,  through  which  threads,  tied  to  the  frog's  toes,  are 
passed  and  held  firmly  by  small  wooden  pins.  A  series  of  holes 
are  also  bored  on  each  side  and  cut  out  at  the  edge,  so  that  it  is 
unnecessary  to  pass  the  twine  through  the  holes,  as  it  may  be 
readily  slipped  into  them.  The  frog  may  be  enclosed  in  a  bag, 
one  foot  being  left  out,  but  a  simpler  and  better  plan  is  to 
swathe  him  in  a  strip  of  muslin  two  inches  wide  and  eight  to 
twelve  inches  long.  The  muslin  is  dipped  in  water,  and  the 


140 


SELECTION  AND    USE 


|i;i:;,;:'      :.        . , 

Pig-  38,  FEOG  FLATE,  PLAN  AND  SECTION 


Fig.  39, 


OF   THE  MICROSCOPE..  141 

frog  rolled  up  in  it  and  laid  on  the  plate,  where  he  is  held  by  a 
few  turns  of  light  packing  twine  passed  into  the  slits  in  the  side 
of  the  plate  and  carried  from  one  to  the  other  and  over  the 
animal.  Small  frogs  are  best  for  this  purpose,  but  when  too 
small  they  are  not  easily  handled.  The  position  of  the  animal 
on  the  plute  is  so  arranged  that  the  foot  may  be  spread  over  the 
glass  plate  that  fills  the  large  opening. 

The  plate  is  attached  to  the  stage  as  follows:  A  cylindrical 
brass  block  (Fig.  40)  is  provided — this  block  having  a  milled  belt, 
which  renders  it  more  easily  turned.     The  upper  surface  of 
this  block   receives  a  screw  which   passes 
through  a  slot  of  considerable  length,  cut  in 
the  frog  plate,  thus  allowing  a  wide  range  of 
motion  on  the  part  of  the  latter;  the  under 
surface  of  the  block  receives  a  second  screw, 
which  serves  to  secure  it  to  the  stage  of  the 
microscope,  as  shown  in  Fig.  39.   The  holes 
for  these  screws  are  not  in  the  same  line, 
their  axes  being  about  a  quarter  of  an  inch 
Fig.  40.  '  apart,  and  the  consequence  is  that  when  the 

brass  block  is  rotated  on  the  stage,  the  screw 
that  passes  through  the  plate  acts  like  a  crank  in  relation  to 
the  plate,  and  moves  it  longitudinally,  provided  it  (the  plate)  is 
kept  from  rotating  with  the  block.  The  upper  screw  is  inserted 
with  sufficient  tightness  to  keep  the  plate  from  shaking,  but  is 
left  so  loose  that  the  plate  can  be  readily  moved  back  and  forth. 
Hence,  while  the  plate  is  attached  to  the  stage  in  such  a  way 
that  it  can  not  tip  up  or  fall  off,  it  may  readily  be  mo\ed  in  two 
directions,  one  the  arc  of  a  comparatively  large  circle,  and  the 
other  a  longitudinal  motion  at  right  angles  to  this. 

This  frog  plate  forms  in  fact  a  sort  of  mechanical  stage  which 
admits  of  very  delicate  movements  being  very  steadily  made. 
Where  this  plate  is  used,  the  microscope  may  be  inclined  to 
any  angle,  and  no  jerking  or  starting  of  the  animal  can  displace 
the  portion  of  the  foot  that  is  under  observation.  Difi'erent 
parts  of  the  same  foot  and  different  corresponding  parts  01  dif- 
ferent feet  are  more  or  less  suited  to  purposes  of  observation, 
according  as  they  are  more  or  less  transparent  and  more 
or  less  fully  supplied  with  vessels.  It  is  therefore  of  great  ad- 


142  SELECTION    AND   TJSE 

vantage  to  be  able  to  select  that  part  which  answers  our  pur- 
pose most  perfectly,  and  this  plate  affords  peculiar  facilities  for 
effecting  this. 

Table.— The  table  used  for  supporting  the  microscope 
should  be  firm  and  substantial,  so  that  all  shake  and  vibration 
maybe  avoided.  Those  who 'use  very  high  powers,  and  desire 
to  avoid  vibration  as  much  as  possible,  will  find  that  a  barrel  or 
box,  filled  with  sand,  and  resting  on  three  feet,  makes  the  best 
support.  Some  years  ago,  having  some  rather  delicate  investi- 
gations to  make,  we  constructed  a  table  in  this  way,  and  found 
the  results  very  gratifying.  Our  table  was  arranged  as  follows: 
a  common  barrel,  cut  down  a  little,  and  filled  with  sand,  was 
supported  on  three  stout  blocks  nailed  to  the  bottom.  The 
table  proper  was  made  of  plank,  nearly  square,  and  it  entirely 
covered  the  top  of  the  barrel.  It  was  supported  by  a  -{-  shaped 
piece  of  wood,  which  was  fastened  to  the  centre  of  the  table, 
and  descended  into  the  sand.  With  such  a  table,  walking  on 
the  floor,  and  the  passage  of  heavy  teams  in  the  street,  produce 
no  vibration,  though,  on  an  ordinary  table,  they  render  work 
with  high  powers  almost  impossible. 

Where  several  persons  wish  to  look  through  the  same  micro- 
scope, it  is  very  awkward  if  each  one  has  to  get  up  and  go  to 
the  instrument.  At  the  same  time  it  is  of  course  impossible  to 
move  the  microscope  without  moving  the  arrangement  for  illu- 
mination also.  This  difficulty  has  been  avoided  by  means  of 
revolving  tables,  around  which  the  observers  sit,  each  one  in 
turn  examining  the  object,  as  the  microscope  is  passed  round  to 
him.  This  is  a  very  excellent,  but  a  somewhat  expensive  ar- 
rangement. The  same  end  may  be  attained  by  placing  the  mi- 
croscope, lamp,  etc.,  on  a  smooth  board  of  suitable  size  and 
shape,  and  passing  this  board  to  each  observer  in  turn.  The 
board,  carrying  microscope,  lamp,  etc.,  may  be  made  to  slide 
quite  easily,  and  if  placed  on  three  feet,  it  is  tolerably  steady. 
Such  a  support,  however,  is  not  to  be  chosen  where  the  micro- 
scope is  used  for  scientific  investigations. 


OP  THE   MICROSCOPE. 


143 


Fig.  41.— STRAIGHT  NOSE-PIECE. 


Double  Ko*e-Piece.— This  is  one  of  the  most  useful 
accessories  that  the  microscopist  can  possess.  The  result  to  be 
obtained,  and  the  method  of  accomplishing  it  are  obvious.  The 

nose-piece  screws  on  to  the 
nose,  or  lower  end  of  the  body 
of  the  microscope,  and  is 
fitted  to  receive  two  objec- 
tives of  different  powers, 
either  one  of  which  may  be 
brought  into  action  by  simply 
turning  the  nose-piece.  In 

this  way  a  low  power  may  be  used  for  finding  objects  and  ex- 
amining them  as  a  whole,  while  the  details  may,  without 
trouble,  be  subjected  to  an 
object-glass  of  much  higher 
power.  Two  forms  of  the 
nose-piece  are  in  use.  The 
older  form  is  straight,  as  in 
Fig.  41;  the  later  form  is 
bent,  as  in  Fig.  42.  The  lat- 
ter form  is  altogether  the  most 
convenient.  Nose-pieces  ca- 
pable of  receiviDg  three  or 
four  ol  >jectives  have  been  con- 
structed, and  a  very  old  mi- 
croscope, at  one  time  in  our 
possession,  had  a  nose-piece 
with  eight  objectives!  The 
modern  nose-piece,  so  ar- 
ranged as  to  be  capable  of 

carrying  ihe  best  objectives,  iz  Fi&-  42.— BEST  NOSE-PIECE. 

the  invention  of  Mr.  Brookes. 


ILLUMINATION-SOUECES  OF  LIGHT. 

Sun  Liglit.— It  is  generally  acknowledged  that  the  best 
light  for  microscopical  purposes  is  that  of  the  sun;  not  direct 
sunlight,  however,,  for  this  is  altogether  too  intense,  but  sun 


144  SELECTION  AND    USE 

light  reflected  from  a  white  wall,  or  a  light  fleecy  cloud.  Sun- 
light is  something  which  we  cannot  command  at  will,  and,  there- 
fore, the  microscopist  can  do  nothing  more  than  select  the  lo- 
cation of  the  room  which  he  occupies.  In  general  a  room  with 
a  northern  aspect  is  to  be  preferred;  if  there  should  also  be 
windows  looking  towards  the  east  or  west,  so  much  the  better, 
provided  they  can  be  completely  darkened  when  not  in  use,  as 
cross  lights  produce  a  bad  effect. 

Artificial  Light.— While  good  daylight  is  the  best  source 
of  illumination,  poor  daylight  is  one  of  the  worst,  and  we  have 
frequently,  during  the  day,  obtained  by  the  use  of  lamps  and 
candles,  results  which  could  not  possibly  be  secured  by  natural 
daylight.  At  the  present  time,  gas-light,  lamp-light  and  can- 
dle light  are  the  most  available  means  of  artificial  illumination. 
Candles  are  rarely  used  except  when  the  microscopist  is  travel- 
ing, or  in  a  peculiar  situation,  but  a  good  candle  gives  very  fair 
results,  especially  if  the  flame  be  protected  from  currents 
of  air,  which  may  easily  be  done  by  extemporizing  a 
shade  out  of  a  piece  of  glass  tube  or  small  lamp  chim- 
ney. Wax,  paraffine  or  sperm  candles  should  be  chosen,  as 
they  give  a  clear,  white  flame.  Common  tallow  candles  give  a 
dull  yellow  flame  of  inferior  quality.  Gas-light,  as  obtained 
from  the  ordinary,  flat,  unprotected  burner,  is  not  sufficiently 
steady;  it  flickers  and  changes,  and  for  microscopy  this  is  the 
worst  fault  that  an  artificial  light  can  have.  Where  gas  is  em- 
ployed it  is,  therefore,  necessary  to  use  an  argand  burner,  with 
a  glass  chimney.  Light  obtained  in  this  way  is  in  general 
very  excellent.  But  the  most  convenient,  as  well  as  the  best 
means  of  illumination,  is  a  good  lamp,  of  which  the  ordinary 
student's  lamp  is,  on  the  whole,  perhaps  the  best  kind.  It 
gives  a  pure,  steady  and  intense  light;  it  is  easily  regulated, 
both  as  regards  brightness,  and  also  position,  and  conse- 
quently direction,  and  it  may  easily  be  procured  almost  any- 
where. In  default  of  a  good  student's  lamp,  any  of  the  ordi- 
nary lamps  with  circular,  or  flat  wicks,  may  be  made  to  answer. 
Where  a  large  quantity  of  light  is  required,  as  in  the  illumina- 
tion of  large  opaque  objects,  the  circular,  hollow  wick,  from 
the  superior  brightness  and  whiteness  of  the  light,  is  always  to 


OF  THE  MICROSCOPE.  145 

be  preferred.  But  where  a  small  light  of  great  intensity  is 
needed,  the  common  flat  -wick,  turned  edgewise  to  the  mirror, 
answers  very  well.  It  is  a  curious  fact  that  flame  is  transparent 
to  light,  and,  therefore,  the  greater  the  depth  of  flauie,  tho 
more  intense  is  the  light.  This  is  easily  tested  by  looking  at 
the  flame  of  a  common  hand  lamp  sidewise  and  edgewise.  lu 
the  latter  case  the  eye  receives  the  light  from  the  entire  flame 
concentrated  to  a  mere  band. 

Several  varieties  of  lamps  have  been  devised  specially  for  the 
use  of  microscopists,  and  some  of  them  are  very  excellent, 
the  most  perfect  being  that  devised  by  Dr.  DrysJale  and  Rev. 
W.  H.  Dallinger,  and  described  in  the  Montldy  Microscopical 
Journal  for  April,  1876. 

It  is  hardly  necessary  to  say  that  all  kinds  of  oil  have  been 
displaced  by  the  mineral  oils  ordinarily  called  kerosene. 

Very  intense  light,  such  as  that  from  burning  magnesium, 
the  calcium  light,  the  Bude  light  and  others,  have  been  tried, 
but  without  material  advantage.  Many  years  ago,  we  arranged 
a  common  kerosene  lamp,  so  that  the  air  surrounding  the  flame 
could  be  enriched  with  a  supply  of  pure  oxygen  when  neces- 
sary. Dr.  Beale  describes  the  same  thing  in  his  work,  but  does 
not  seem  to  regard  it  as  of  any  advantage.  When  used  as  a 
source  of  direct  light,  however,  we  found  that  it  more  nearly 
resembled  sunlight  than  any  other  artificial  source  of  illumina- 
tion. A  large  diaphragm  or  shade,  with  an  aperture  of  mod- 
erate size,  was  placed  close  to  the  light,  which  was  placed  at 
some  distance  from  the  microscope,  and  the  rays  passed  di- 
rectly through  the  object,  not  being  reflected  from  a  mirror. 
The  results  in  some  cases  were  well  worth  the  trouble  incurred. 
It  is  probable  that  in  some  cases  very  excellent  results  could  be 
obtained  from  the  electric  light  if  properly  arranged.  This, 
however,  is  a  department  of  microscopy  which  is  certainly  not 
suited  to  beginners,  and  we,  therefore,  dismiss  it. 

The  rays  of  light,  from  whatever  source  obtained,  are  either 
parallel,  convergent  or  divergent;  and  in  the  illumination  ot 
transparent  objects  the  character  of  the  light,  as  depending 
upon  these  features,  is  of  marked  importance.  This  subject, 
and  the  action  of  lenses  and  mirrors  in  changing  the  relative 
direction  of  the  rays,  should  be  carefully  studied  by  the  stu- 


146  SELECTION  AND  USE 

dent,  who  will  find  it  fully  discussed  in  any  work  on  optics. 
The  general  principles  may  be  best  explained  by  a  few  experi- 
mental illustrations. 

Take  a  piece  of  cardboard  about  six  inches  square,  and  in  it 
punch  a  hole  about  half  an  inch  in  diameter.  If  this  card  be 
held  in  front  of  a  wall  upon  which  the  sun  is  shining  strongly, 
we  will  see  the  shadow  of  the  card  and  a  round  spot  of  light 
exactly  the  size  of  the  hole.  If  the  card  be  now  moved  away 
from  the  wall,  the  shadow  and  the  bright  spot  will  still  remain 
of  the  same  size,  showing  clearly  that  the  rays  proceeding  from 
the  sun  are  sensibly  parallel.  The  same  holds  true  of  a  bright 
cloud  or  a  white  wall  placed  at  a  great  distance;  but  when  the 
wall  or  other  reflecting  object  is  very  near,  the  rays  no  longer 
possess  this  character  to  the  same  extent. 

If  in  the  first  experiment  the  wall  be  illuminated  by  a  candle 
instead  of  by  the  sun,  it  will  be  found  that  as  the  card  is  moved 
from  the  wall  the  shadow  and  the  spot  become  larger,  showing 
that  the  rays  are  divergent  instead  of  parallel.  The  same  effect 
is  produced  by  fixing  both  the  lamp  and  the  card  on  a  stand 
and  moving  them  away  from  the  wall. 

Convergent  rays,  that  is  rays  that  tend  to  meet  at  a  point, 
can  be  obtained  only  by  passing  parallel  or  divergent  rays 
through  a  lens,  or  reflecting  them  from  a  concave  mirror.  By 
carefully  arranging  a  large  convex  lens  in  the  path  of  rays  that 
are  divergent,  it  is  easy  to  render  them  parallel.  They  are 
known  to  be  parallel  when  the  bright  spot  which  they  make  on 
a  fixed  surface,  after  passing  through  a  hole,  is  not  varied  in 
size  by  changing  the  position  of  the  hole. 

The  variations  which  are  produced  in  the  appearances  of 
objects  when  they  are  viewed  by  light  possessing  these  different 
characteristics  c^in  only  be  learned  by  practice,  and  the  young 
microscopist  should  experiment  in  every  conceivable  way. 

Whatever  be  the  source  of  light  employed,  most  objects  may 
be  viewed  by  means  of  any  one  of  several  very  different 
methods.  Thus,  an  object,  if  transparent,  may  be  viewed  by 
transmitted  light,  thati-s,  by  light  reflected  from  the  mirror,  and 
passing  through  the  object.  Ir  opaque,  it  may  be  viewed  by  re- 
flected\ig\iiy  in  which  case  the  light  that  passes  to  the  eye  through 
the  microscope  is  reflected  from  the  surface  of  the  object, 


OF  THE   MICROSCOPE. 


147 


ILLUMINATION  OP  OPAQUE  OBJECTS. 

Diffused  Ligli;.— This  term  is  applied  to  ordinary  day- 
light or  lump-light,  allowed  to  fall  on  the  object  without  the 
intervention  of  any  special  means  of  concentration.  That  dif- 
fused light  may  be  available  for  the  illumination  of  objects,  it 
is  necessary  that  the  objectives  be  good.  Oujects  which,  with 
ordinary  triplets  of  low  angular  aperture,  are  entirely  invisible, 
become  beautifully  distinct  when  a  better  class  of  objectives  is 
used.  Under  favorable  circumstances  the  view  obtained  in  this 
way  of  any  well  marked  object  is  very  pleasant. 

Bulls-Eye  Condenser.— This  is  a  large  lens  of  compar- 
atively short  focus,  which  is  made  to  condense  the  light  on  the 
object  in  the  same  way  that  the  common  burning-glass  acts,  but 
with  effects  greatly  less  marked,  since  the  light  is  so  much  less 
intense.  In  some  cases  the  condensing  lens  is  attached  to  the 
microscope,  and  in  some  special  cases  this  is  very  convenient, 
but  where  there  is  only  one  condenser,  it  should  be  mounted  on 
a  stand,  as  shown  iu  Fig.  43,  so  that  it  may  be 
placed  at  any  height  and  turned  in  any  direc- 
tion. Placed  between  the  object  and  the 
lump,  it  collects  the  rays  of  the  latter  to  a, 
focus  which  brightly  illuminates  any  object 
upon  which,  it  may  fall.  Opaque  objects, 
which  by  diffused  light  are  barely  visible 
under  the  microscope,  become  very  distinct 
and  clearly  denned  when  thus  illuminated, 
and  many  of  them,  such  as  the  wings  of  in- 
sects and  certain  minerals,  appear  in  the  most 
gorgeous  colors,  which,  however,  are  perfectly 
natural,  and  are  not  the  result  of  chromatic 
defects  in  the  lenses. 

Iu  viewing  an  opaque  object  by  reflected 
light,  it  is  evident  that  we  are  enabled  to 
judge  of    the    irregularities  of    the   surface 
largely  by  means  of  the  shadows  cast  by  the 
prominences.   By  raising  or  lowering  the  lamp,  and  also  the  con- 


Fig.  43.— BUIXS-EYE 
CONDENSER. 


148  SELECTION   AND    USB 

denser,  the  direction  and  extent  of  these  shadows  may  be  greatly 
varied.  Hence  one  of  the  advantages  of  the  students'  lamp. 

An  important  use  of  the  condensing  lens  is  to  change  the 
direction  or  character  of  the  rays  employed.  Thus,  -when  a 
lamp  is  in  use  the  rays  are  divergent,  and  the  easiest  way  to 
render  them  parallel  is  to  pass  them  through  a  condensing  lens. 
To  effect  this  the  distance  of  the  lens  from  the  lamp  must  be 
exactly  the  same  as  that  at  which  it  brings  parallel  rays  to  a 
focus.  In  other  words,  the  lens  must  be  at  a  distance  from  the 
lamp  which  is  exactly  equal  to  its  focal  distance  for  parallel  rays. 

Condensing  lenses  are  made  of  all  sizes,  and  some  of  them 
are  quite  expensive,  but  we  have  frequently  obtained  wonder- 
fully fine  results  by  means  of  a  cheap  lens  of  small  size,  but 
good  form.  A  condensing  lens  is,  perhaps,  the  most  important 
accessory  that  can  accompany  a  microscope. 

Side  Reflector. — This  is  a  small  silvered  concave  mirror, 
which  is  used  to  throw  the  light  on  the  object  for  the  same  pur- 
pose as  the  condensing  lens.  The  results  which  it  gives  are 
slightly  different,  and  it  is  a  most  valuable  means  of  illumina- 
tion. It  has  not  been  so  generally  introduced  as  it  deserves  to 
be,  and  few  microscopes  are  furnished  with  it  unless  to  special 
order.  It  should  always  be  used  in  combination  with  a  bulls-eye 
condenser,  as  light  of  much  greater  intensity  is  thus  obtained. 

Tlie  Lrfieberlmlm.— This  was  one  of  the  first  instruments 
used  for  illuminating  opaque  objects.  It  consists  of  a  small, 
concave,  spherical  mirror,  through  the  centre  of  which  the  ob- 
jective passes,  the  focus  of  the  mirror  and  objective  coinciding. 
The  object  must  be  small,  and  is  generally  mounted  on  a  small 
circular  disc  of  leather  or  card,  which  stops  out  the  central 
rays,  while  the  light  which  passes  round  it  strikes  against  the 
concave  mirror,  and  is  reflected  back  again  upon  the  object. 

The  Lieberkuhn  gives  very  brilliant  effects  with  many  ob- 
jects, and  if  well  managed  it  enables  us  to  obtain  very  satisfac- 
tory views,  especially  with  powers  which  are  too  high  to  admit 
the  use  of  the  side  reflector,  the  parabola,  or  the  bulls-eye. 
The  great  objection,  however,  is  the  fact  that  the  light  falls 
almost  vertically,  thus  obliterating  all  the  shadows  pro- 


OF  THE   MICROSCOPE. 


149 


cl need  by  elevations  on  the  object.  For  "show"  objects  the 
Lieberkuhn  is  unequalled  in  its  effects,  and  there  is  no  doubt 
that  in  certain  investigations  it  may  be  made  to  do  good  ser- 
vice. 

The  Parabolic  Reflector.— This  accessory  was  first 
made  by  Messrs.  Beck  for  Mr.  Sorby,  who  employed  it  to  ex- 
amine the  microscopical  structure  of  iron  and  steel.  As  ordin- 
arily constructed,  it  consists  of  a  parabolic  mirror  attached  to 
the  end  of  a  rod  furnished  with  universal  joints,  so  that  it  may 
be  placed  in  any  position  as  regards  the  object  and  the  illumin- 
ation. It  answers  admirably  for  condensing  the  light  on  the 

surface  of  objects,  and  by  throw- 
ing the  rays  in  any  particular  di- 
rection across  the  surface,  the 
observer  is  enabled,  by  means  of 
the  shadows,  to  determine  the 
nature  of  irregularities  upon  some 
objects  in  a  very  satisfactory  man- 
ner. In  this  it  resembles  the  or- 
dinary side  reflector,  which,  how- 
ever, is  formed  to  a  spherical  in- 
stead of  a  parabolic  curve.  Like 
the  side  reflector,  this  illuminator 
should  always  be  made  to  receive 
parallel  rays,  and  condense  them 
upon  the  object.  To  obtain  par- 
Fig.  44.— PARABOLIC  REFLECTOR.  allel  rays,  place  the  lamp  in  the 

focus  of  the  bulls-eye  condenser. 

The  side  and  the  parabolic  reflectors  cannot  be  used  with  ob- 
jectives which  have  a  short  working  distance,  since  the  rays 
from  the  lamp  must  reach  the  reflector  from  the  opposite  side 
of  the  objective. 

Since  many  stands  have  no  conveniences  for  attaching  this 
accessory,  some  opticians  furnish  it  with  an  adapter,  whereby 
it  may  be  fastened  between  the  objective  and  the  nose-piece. 
Such  a  contrivance,  however,  must  have  originated  in  a  want  of 
knowledge  of  the  principles  which  control  this  method  of  illum- 
ination. It  is  evident  that  the  object  should  always  be  in  the 


150  SELECTION  AND  USE 

focus  of  the  reflector;  if,  in  adjusting  the  focus  of  the  objective, 
\ve  move  the  reflector,  we  must,  of  course,  destroy  the  proper 
relation  of  the  latter  to  the  object. 

Objectives  with  Tapered  Fronts.— When  the  objec- 
tive has  a  very  broad  front  and  a  short  working  distance,  as  is 
the  case  with  most  objectives  of  high  angles,  it  is  impossible 
to  illuminate  the  object  satisfactorily  with  the  bulls-eye  con- 
denser. To  avoid  this  difficulty,  several  makers,  notably  Mr. 
Swift,  of  London,  have  narrowed  the  brass  work  of  their  fronts 
to  the  last  limit,  so  as  not  to  interfere  with  the  illumination  of 
opaque  objects.  Mr.  Tolles  has,  however,  gone  a  step  further, 
and  has  allowed  the  glass  of  his  front  lenses  to  project  beyond 
the  brass  work,  so  that  the  bull's-eye  may  be  used  with  com- 
paratively high  powers.  We  have  a  one-quarter  constructed  on 
this  principle,  which  allows  of  the  clearest  illumination,  and 
shows  the  P.  angulatum  as  an  opaque  object  so  well  that  the 
markings  are  sharp  and  well  defined.  These  lenses  are  mode- 
rate in  price,  very  easily  used,  and  give  very  satisfactory 
results. 

Smith's  Vertical  Illuminator.— This  admirable  de- 
vice is  due  to  Prof.  Hamilton  L.  Smith,  of  Hobart  College,  Ge- 
neva, N.  Y.,  and  is  intended  for  use  with  objectives  of  such  high 
power,  that  the  Lieberkuhn,  condensing  lens,  side  reflector, 
etc.,  cannot  be  employed.  Several  different  arrangements  have 
been  suggested.  The  first  was  a  small  annular  silver  reflector, 
placed  just  above  the  back  lenses  of  the  objective,  and  forming 
an  angle  of  45°  with  the  optic  axis  of  the  microscope.  A  hole 
in  the  side  of  the  brass  mounting  of  the  objective  admitted  the 
light,  which  was  thus  thrown  down  through  the  lenses  on  to 
the  object,  and  back  again  to  the  eye.  We  have  used  such  an 
arrangement  with  most  satisfactory  results.  For  example,  with 
a  one-fourth  inch  objective,  thus  fitted,  it  is  easy  to  view  the 
P.  angulatum  as  an  opaque  object,  and  bring  out  its  markings. 
The  illuminator,  as  thus  constructed,  may  be  either  a  separate 
reflector  which  may  be  screwed  into  the  nose-piece  of  any  mi- 
croscope, and  which  is  furnished  at  its  lower  end  with  the  So- 
ciety screw,  into  which  the  objective  is  inserted,  or  it  may  be 


OF   THE   MICROSCOPE.  151 

a  permanent  part  of  the  particular  objective  employed,  and  be 
specially  adapted  thereto.  In  our  own  experience,  the  best 
results  have  been  obtained  by  the  latter  arrangement. 

Instead  of  the  silver  reflector,  Mr.  B.  Beck  uses  a  thin  glass 
plate  (an  ordinary  cover-glass),  which  is  inserted  into  an  adapter 
which  fits  between  the  nose-piece  and  the  objective.  The  thin 
glass  is  supported  by  a  small  pin  with  a  milled  head,  by  which 
it  may  be  turned  so  as  to  present  its  surface  at  the  tyest  angle 
for  reflecting  downward  the  light  admitted  through  a  suitable 
aperture. 

All  forms  of  the  vertical  illuminator  give  their  best  results 
when  used  with  immersion  objectives,  as  has  been  very  fully 
shown  by  Mr.  Geo.  "W.  Morehouse,  of  Way  land,  N.  Y.,  who 
uses  with  success  objectives  as  high  as  the  one-tenth.  The 
markings  on  the  most  difficult  tests — even  the  19th  band  of 
Nobert's  plate — are  shown  clearly  and  well  by  this  arrange- 
ment. 

In  using  the  vertical  illuminator,  success  will  depend  greatly 
upon  the  management  of  the  illumination.  The  size  of  the 
aperture  which  admits  the  light  to  the  reflector  should  be 
carefully  regulated  and  diaphragmed  down,  if  necessary,  and 
the  rays  should  be  rendered  parallel  by  means  of  the  bulls-eye 
condenser. 

Tolles'  Vertical  Illuminator.— So  far  as  we  have  been 
able  to  find,  Prof.  H.  L.  Smith  was  the  first  to  illuminate  the 
object  by  light  passed  down  through  the  objective,  and  the 
different  forms  which  have  been  introduced,  such  as  Beck's, 
Powell  &  Lealaud's,  etc.,  are  mere  modifications  of  his  original 
plan.  To  Mr.  Tolles,  however,  is  due  the  invention  of  a  modi- 
fication which  exhibits  considerable  originality.  He  inserts, 
above  the  front  lens,  a  small  prism,  which  is  so  constructed 
that  the  light  passes  ih  freely,  and  is  then  totally  reflected 
downwards.  In  this  way  the  rays  pass  down  through  only  one 
lens  of  the  system,  instead  of  through  all  of  them.  Mr.  W.  A. 
Bogers  has  used  this  illuminator  with  very  satisfactory  results 
in  the  examination  of  fine  rulings  on  metal,  incident  to  his 
investigation  of  the  comparative  value  of  various  standard 
linear  measures. 


152  SELECTION   AND    USE 


ILLUMINATION  OF  TRANSPARENT  OBJECTS. 

The  different  methods  which  have  been  devised  for  viewing 
transparent  objects  are  quite  as  numerous  as  those  available  for 
opaque  ones,  and  require  quite  as  much  tact  and  study.  A 
skilful  worker,  who  thoroughly  understands  the  points  essential 
to  good,  (3r  rather  to  appropriate  and  efficient  illumination,  will 
attain  results  wonderfully  superior  to  those  achieved  by  persons 
ignorant  of  the  subject,  and  this,  too,  although  the  latter  may 
be  working  with  far  superior  instruments.  This  is  seen  every 
season  at  our  microscopical  exhibitions  and  conversaziones,  and 
although  the  work  done  on  these  occasions  is  chiefly  for  show, 
the  same  principle  holds  good  in  regard  to  work  done  in  the 
direction  of  study  and  investigation. 

Direct  and  Reflected  Light.— When  the  microscope 
is  so  arranged  that  the  light  from  a  lamp  or  other  self-luminous 
body  shall  pass  directly  through  the  object  and  into  the  micro- 
scope without  being  first  reflected  from  the  mirror,  the  illumin- 
ation is  said  to  be  direct,  in  distinction  from  light  which  has 
been  first  reflected  from  a  mirror  or  other  surface.  Light  from 
a  cloud  or  a  white  wall  can  scarcely  be  regarded  as  direct. 
Direct  light  gives  results  which  are  appreciably  different  from 
those  produced  by  reflected  light,  since  light  always  suffers  a 
change  in  character  by  reflection.  These  two  kinds  of  illumin- 
ation may  be  either  axial  or  oblique,  and  in  the  case  of  both 
reflected  and  direct  light,  if  the  source  of  light  be  very  distant, 
the  rays  will  be  sensibly  parallel,  but  if  the  source  of  light  be 
very  near,  the  rays  will  be  divergent,  and,  consequently,  under 
such  circumstances,  the  illumination  must  in  part  be  more  or 
less  oblique. 

Axial  or  Central  Liglit.— When  the  mirror,  either 
plane  or  concave,  is  placed  directly  in  the  axis  of  the  micro- 
scope, and  reflects  the  light  through  the  tube,  the  illumination 
is  said  to  be  axial  or  central.  The  same  term  also  applies  to 
direct  light,  when  the  direction  in  which  the  rays  pass  through 
the  object  coincides  with  the  optical  axis  of  the  instrument. 


OF  THE   MICROSCOPE.  153 

The  rays  must,  of  course,  be  parallel.  If  either  divergent  or 
convergent,  some  of  the  rajs  •will  be  oblique.  Purely  axial  or 
central  illumination  can  be  obtained  only  by  passing  the  light 
through  a  very  small  hole  placed  some  distance  below  the  stage. 

Oblique  Light. — Many  objects  fail  to  show  their  peculi- 
arities when  illuminated  by  parallel  rays  of  light  passing 
through  them  in  the  direction  of  the  optic  axis  of  the  micro- 
scope, but  are  seen  very  clearly  when  the  light  is  sent  through 
them  obliquely.  To  secure  illumination  by  oblique  light  re- 
flected from  the  mirror,  the  latter  must  be  so  suspended  that  it 
can  be  turned  to  one  side,  and  thus  send  a  beam  of  light  through 
the  object  at  an  acute  angle.  "Where  direct  light  is  employed, 
the  necessary  degree  of  obliquity  may  be  obtained  by  adjusting 
the  position  of  the  lamp — a  device  to  which  we  have  resorted 
when  compelled  to  use  a  stand  in  which  the  mirror  did  not 
swing  to  one  side.  In  this  way,  also,  oblique  light  may  be  em- 
ployed to  illuminate  objects  viewed  through  a  pocket  lens,  and 
very  interesting  effects  obtained.  For  the  resolution  of  fine 
markings  upon  diatoms,  etc.,  oblique  illumination  is  a  neces- 
sity. When  the  angular  aperture  of  the  objective  is  low,  and 
the  light  is  very  oblique,  the  objects  appear  light  on  a  dark 
ground — in  fact  a  sort  of  dark  ground  illumination  is  obtained. 

The  Achromatic  Condenser.— The  earlier  forma  of 
the  achromatic  condenser  consisted  simply  of  an  achromatic 
lens,  similar  to  an  object-glass,  so  arranged  that  by  means  of  it 
the  light  from  the  mirror  could  be  brought  to  a  focus  on  the 
object.  With  some  objects,  even  this  simple  contrivance  gave 
very  fine  results.  It  was  soon  found,  however,  that  great  ad- 
vantage was  derived  from  cutting  off  portions  of  the  pencil  of 
rays  transmitted  by  the  condenser,  and  by  means  of  the  proper 
diaphragms,  central,  peripheral  and  one-sided  or  oblique  illum- 
ination was  obtained.  First-class  achromatic  condensers  be- 
came, therefore,  quite  complicated  and  expensive.  Several 
cheaper  but  very  efficient  forms  are  now  made  by  opticians,  a 
favorite  being  the  WeVjster  condenser,  shown  in  Fig.  45. 

Of  this  accessory  Carpenter  gives  the  following  very  practical 
description:  "In  its  present  form  the  arrrangement  of  the 


154  SELECTION  AND    USE 

lenses  strongly  resembles  that  used  in  the  Kellner  Eye-piece; 
the  field-glass  of  the  latter  serving  as  a  condenser  to  receive 
the  cone  of  rays  reflected  upwards  from  the  mirror,  and  to 

make  it  converge  upon 
a  smaller  achromatic 
combination,  which 
consists  of  a  double- 
convex  lens  of  crown, 
with  a  plano-convex 
Fig.  45. — WEBSTER  CONDENSER.  lens  of  flint,  the  plane 

side  of  the  latter  be- 
ing next  the  object.  These  lenses  are  of  large  size  and  deep 
curvature;  so  that  when  their  central  part  is  stopped  out,  the 
rays  transmitted  from  their  peripheral  portion  meet  at  a  wide 
angle  of  convergence,  and  have  the  effect  of  those  transmitted 
through  the  peripheral  portion  of  the  ordinary  achromatic 
condenser.  When,  on  the  other  hand,  this  combination  is  used 
with  a  diaphragm  that  allows  only  the  central  rays  to  pass, 
these  rays  meet  at  a  small  angle;  and  the  illumination  thus  given 
is  very  suitable  for  objects  viewed  with  low  powers.  Again,  by 
stopping  out  the  central  portion  of  the  combination,  and  re- 
moving the  condenser  to  a  short  distance  beneath  the  object, 
the  effect  of  a  black  ground  illumination  can  be  very  satisfac- 
torily obtained  with  objectives  of  moderate  angular  aperture. 
Further,  by  stopping  out  not  only  the  central,  but  also  a  great 
part  of  the  peripheral  rays,  so  as  only  to  allow  the  light  to  enter 
from  a  small  portion  or  portions  of  the  margin,  oblique  illumin- 
ation can  be  most  effectively  obtained." 

The  Wenliam  Reflex  Illuminator.— This  is  gener- 
ally conceded  to  be  the  most  perfect  device  for  illuminating 
balsam-mounted  objects  when  viewed  by  objectives  of  very 
high  angles.  It  is  shown  in  section  in  Figure  46,  where  a  is  a 
cylinder  of  glass  half  an  inch  long,  and  four-tenths  in  diameter, 
the  lower  convex  surface  of  which  is  polished  to  a  radius  of 
four-tenths.  The  top  is  flat  and  polished.  Starting  from  the 
bottom  edge,  the  cylinder  is  worked  off  to  a  polished  face  at  an 
angle  of  64°.  Close  beneath  the  cylinder  is  set  a  plano-convex 
lens  of  li  inch  focus.  Parallel  rays,/,/,/,  sent  through  the 
lens,  after  leaving  the  lower  convex  surface  of  the  cylinder, 


OF   THE   3UCKOSCOPE.  155 

would  be  refracted  to  the  point,  h,  if  continued  in  solid  glass, 
but  by  impinging  on  the  inclined  polished  surface  (which  is 
far  within  the  angle  of  total  reflection),  they  are  thrown  on  the 
flat  segmental  top  ;  here  they  would  be  totally  reflected  and 
beaten  down  again  to  a  point  outside  the  cylinder,  but  if  an 


Fig.  46. 

object-slide,  c,  be  laid  over  the  flat  top  with  an  intervening 
film  of  water,  the  rays  will  proceed  on  to  the  point,  g,  if  the 
glass  slide  be  thick  enough.  As  shown  in  the  engraving,  how- 
ever, the  slide  is  of  the  usual  thickness,  and  the  point,  g,  lies 
above  it ;  therefore,  if  the  upper  surface  of  the  slide  is  clean 


156  SELECTION  AND   USE 

and  polished,  the  rays  will  be  totally  reflected  therefrom,  and 
will  be  sent  to  the  point,  b,  outside  the  cylinder.  If,  however, 
any  insect  scales,  diatoms,  or  thin  flat  objects  should  be  in 
contact  with  the  upper  surface  of  the  slide,  the  rays  will  be 
enabled  to  pass  through,  and  the  objects  will  be  brilliantly 
lighted  up  on  a  dark  ground.  Objects  seen  in  this  way  show 
many  points  of  their  structure  with  remarkable  clearness. 
Speaking  of  the  Amphipleura  pettucida,  Mr.  "Wenham  tells  us 
that  it  assumed  a  substantial  appearance  not  seen  in  any  other 
way,  and  at  once  displayed  its  striae  with  an  £th  that  had  never 
resolved  them  before. 

It  is  very  evident  that  the  results  which  we  have  just  de- 
scribed will  only  take  place  when  a  stratum  of  air  lies  above 
the  slide,  and  the  objects  which  we  wish  to  view  must  be  in 
contact  with  the  slide,  and  not  mounted  on  the  cover,  as  is 
usually  the  case.  But  if  the  objects  be  mounted  in  balsam, 
the  light  will  pass  through  to  the  upper  surface  of  the  cover, 
whence  it  will  be  totally  reflected  down,  and  the  objects  will 
be  invisible,  or,  if  sufficiently  substantial,  will  appear  on  a 
black  ground  with  dry  objectives.  But  if  we  use  an  immersion 
objective,  the  light  will  no  longer  be  totally  reflected  at  the 
surface  of  the  cover,  but  will  pass  through,  and  will  be  taken 
up  by  the  objective  provided  the  latter  has  sufficient  angular 
aperture. 

Used  under  these  latter  conditions,  the  reflex  illuminator 
constitutes  a  means  of  obtaining  oblique  illumination  of  great 
purity  and  force,  and  the  instrument  is  now  a  general  favorite 
for  this  purpose,  for  which  it  was  first  used  by  Mr.  Samuel 
Wells,  of  Boston. 

To  use  the  reflex  illuminator  successfully  demands  great  care 
and  some  experience,  but  those  who  will  carefully  study  its 
•  construction  and  mode  of  action,  will  find  no  difficulty  in 
getting  good  results.  First  of  all,  then,  we  must  remember 
that  the  reflex  acts  as  a  condenser,  and  consequently  it  is 
necessary  to  use  but  a  moderate  amount  of  light,  which  should 
be  as  intense  as  possible.  This  is  best  obtained  from  a  small 
hand  lamp.  Secondly,  we  must  use  parallel  rays,  or  the  points 
to  which  the  light  is  brought  to  a  focus  will  not  be  those  which 
accord  with  the  other  features  of  the  instrument.  Parallel  rays 


OP  THE   MICROSCOPE.  157 

may  be  either  the  light  from  a  bright  cloud  thrown  up  by  the 
plane  mirror  ;  divergent  rays  from  a  lamp  made  parallel  with 
the  concave  mirror ;  or  divergent  rays  from  a  lamp  made 
parallel  with  a  bulls-eye  condenser,  and  thrown  up  by  the 
plane  mirror  ;  or  a  combination  of  these  last.  Thirdly,  we  must 
remember  that  the  instrument  has  two  centres,  arising  from  the 
fact  that  the  optic  axis  is  bent  by  internal  reflection  from 
the  polished  facet  of  the  cylinder.  Hence,  we  have  the  centre  of 
the  lens  below  the  cylinder,  to  which  the  illumination  must  be 
adjusted,  and  we  have  another  centre  at  which  the  rays  are 
brought  to  a  focus,  and  which  must  be  made  to  coincide  with 
the  optic  axis  of  the  microscope.  This  centre  is  generally 
marked  by  the  maker  on  the  small  side  piece,  e,  and  it  should 
be  brought  exactly  to  the  centre  of  the  field  of  view  of  the  ob- 
jective, a  low  powqr — say  a  half  inch  or  two-thirds — being  use.l 
for  this  purpose,  in  the  first  place. 

It  is.  of  course,  obvious  that  by  adjusting  the  other  condi- 
tions of  the  instrument,  the  points  which  we  have  laid  down 
may  be  varied.  Thus,  for  example,  divergent  rays  may  be 
used  ;  but  in  that  case  the  focus  of  the  rays  will  not  be  at  the 
point,  g.  unless  the  flat  top  of  the  cylinder  is  depressed  a  little, 
which  may  be  easily  done  if  a  tenacious  liquid  like  glycerine  be 
used  as  the  connecting  medium. 

We  think  that  if  the  reader  will  bear  these  points  in  mind, 
and  will  work  over  this  accessory  faithfully  for  a  few  hours,  he 
will  attain  results  which  will  abundantly  compensate  him  for 
the  labor  spent. 

The  reader  must  always  bear  in  mind  that  where  totally  re- 
flecting surfaces  are  used,  as  in  paraboloids,  reflex  illuminators, 
etc.,  they  must  be  kept  scrupulously  clean  or  they  lose  their 
effect. 

The  Wenham  Prism.— This  simple  and  ingenious  little 
device  was  first  described  by  Mr.  Wenham  in  a  paper  read 
before  the  Eoyal  Microscopical  Society,  March  26,  1856.  At 
that  time  immersion  objectives  were  unknown,  or  at  least  were 
not  in  use,  and  the  effect  of  the  prism  was  to  produce  a  very 
brilliant  dark-ground  illumination  ;  but,  as  with  the  Keflex 
Illuminator,  the  introduction  of  immersion  objectives  of  wide 


158  SELECTION   AND   USE 

angle  lias  greatly  enlarged  the  scope  of  usefulness  of  this  little 
piece  of  apparatus. 

The  Wenham  prism  consists  of  a  small  right-angled  prism  of 
crown  glass,  which  is  "  patched  "  on  to  the  under  side  of  the 
slide  by  means  of  a  little  glycerine,  oil  of  cloves,  or  any  similar 
fluid.  In  the  figure,  b  is  the  slide  and 
. —  ~  -.  ^  cover,  and  a  is  the  prism.  Rays  of 
'  light*  as  shown  by  the  dotted  lines, 

^  sen^  through  the  face  of  the  prism, 
pass  straight  to  the  upper  surface  of 
Fig.  47.  the  cover,  and  if  a  dry  objective  be 

THE  WENHAM  PRISM.  used,  they  are  reflected  down  again  on 
to  the  underlying  objects  in  balsam, 

which  appear  brilliantly  illuminated,  as  if  beneath  a  speculum. 
An  immersion  object-glass  prevents  the  covering  glass  from 
acting  as  a  speculum,  and  light  emerges  beyond  what  would 
otherwise  be  the  critical  angle,  thus  affording  very  oblique  illu- 
mination, which,  with  suitable  objectives,  enables  us  to  resolve 
the  most  difficult  tests. 

If  the  prism  is  used  with  dry  mounted  objects,  total  reflection, 
instead  of  occurring  from  the  cover,  takes  place  from  the  upper 
surface  of  the  slide  itself,  and  all  is  total  obscurity  except  where 
some  object,  such  as  a  butterfly's  scale  or  a  diatom,  is  in  inti- 
mate contact  with  the  slide,  and  then  these  objects  allow  the 
transmission  of  light,  and  appear  curiously  distinct  in  a  jet- 
black  field. 

This  illuminator  requires  no  fitting  to  adapt  it  to  the  micro- 
scope. It  is  merely  stuck  on  the  under  surface  of  the  slide 
with  a  little  glycerine ;  the  slide  is  then  placed  on  the  stage, 
the  object  found,  and  the  position  of  the  prism  adjusted  by  the 
finger. 

It  has  been  objected  to  this  method  of  attaching  it,  that  when 
the  microscope  is  inclined,  the  prism  is  apt  to  slide  down.  This 
will  not  occur  if  the  quantity  of  liquid  used  be  very  small.  It 
is  not  difficult,  however,  to  arrange  a  very  thin  plate  of  metal 
with  a  square  hole  in  the  centre,  the  metal  which  originally 
filled  the  hole  being  bent  down  so  as  to  form  an  ear  or  clip 
at  each  side  of  the  opening.  These  clips  grasp  the  prism, 
which,  being  cemented  into  this  frame,  may  be  laid  on  the 


OF   THE   MICROSCOPE.  159 

;  over  it  is  placed  the  slide.  If  the  metal  plate  be 
fastened  to  the  stage  by  any  contrivance,  the  object-slide 
may  be  moved  about  at  will  without  disturbing  the  prism. 

As  we  have  already  stated,  the  Wenham  prism  has  a  right 
angle  and  two  angles  of  45°  each.  Prisms  for  the  same  purpose 
have  been  made  with  unequal  angles  at  the  base,  and  with  other 
than  a  right  angle  at  the  apex,  and  this  is  sometimes  advan- 
tageous. 

It  is,  of  course,  obvious  that  the  Wenham  prism  transmits 
the  rays  with  their  direction  unaltered.  If  we  wish  to  condense 
them,  this  must  be  done  previously  by  means  of  mirrors  or 
lenses.  Two  illuminators,  which  combine  the  effects  of  the 
condenser  and  the  prism,  have  been  devised  by  Mr.  Wenham, 
and  we  will  now  describe  them. 

The     Hemispherical    Illuminator.— Instead    of   a 

prism,  Mr.  Wenham  has  used  a  hemispherical  lens,  patched  on 
to  the  slide  in  the  same  manner  as  that  described  for  the  prism. 
This  lens  concentrates  the  rays  on  the  object,  and  as  rays  from 
any  direction  will  always  enter  the  lens  at  right  angles  to  some 
part  of  its  surface,  more  light  and  greater  obliquity  can  be  ob- 
tained by  it  than  by  the  prism.  This  device  was  described  by 
Mr.  Wenham  in  the  same  paper  in  which  he  first  described 
the  right-angled  prism,  but  we  believe  it  was  first  applied 
by  Mr.  Tolles  as  an  oblique  illuminator  for  balsam-mounted 
objects  seen  with  immersion  lenses. 

The  u  Half-Button."— A  still  more  efficient  illuminator, 
and  one  which  is  complete  in  itself,  is  shown  in  the  engravings, 
Fig.  48  being  a  side  view,  Fig.  49  a  section,  and  Fig.  50  a  per- 
spective view  of  this  little  contrivance.  It  consists  of  a  semi- 
circular disc  of  glass  of  one-quarter  of  an  inch  radius  ;  the  edge 
is  rounded  and  well  polished  to  a  transverse  radius  of  one-tenth 
of  an  inch,  for  the  reason  that  the  focus  of  a  spherical  surface 
on  crown  glass  falls  within  its  substance  to  nearly  three  times 
the  radius,  consequently  the  line  of  light  will  be  in  the  most 
concentrated  position  at  one-twentieth  of  an  inch  above  the 
centre  of  the  semi-disc,  which  distance  is  sufficient  to  reach  ob- 
jects mounted  on  slides  of  the  usual  thickness.  The  "half- 


160  SELECTION  AND  USE 

button,"  as  it  has  been  called,  is  connected  to  tlie  tinder  side 
of  the  slide  by  means  of  water,  glycerine,  or  oil  of  cloves,  its 
sides  being  grasped  by  a  simple  kind  of  open  clip  attached  to 
the  sub-stage.  This  illuminator  is  complete  in  itself,  and  re- 
quires no  supplementary  condensing  lens ;  the  obliquity  is 


Figs.  48. 


simply  obtained  by  swinging  the  ordinary  mirror  sideways, 
and  by  this  means  Amphipleura  pellucida,  mounted  in  balsam, 
can  be  at  once  resolved. 

The  Woodward  Illuminator.— This  method  of  mount- 
ing and  arranging  a  right-angled  prism  for  the  illumination  of 
balsam-moulted  objects,  was  described  by  Col.  Dr.  Woodward 
in  a  paper  read  before  the  Eoyal  Microscopical  Society,  June  6, 
1877.  It  consists  of  a  prism  of  glass,  the  apex  of  which  has 
been  truncated.  This  prism  is  cemented  to  the  truncated  apex 
of  a  similar  prism  of  brass,  the  long  side  of  the  glass  prism 
being  upward,  and  connected  with  the  under  surface  of  the 
object-slide  by  suitable  liquid  (glycerine,  oil  of  cloves,  etc.) 
The  homologous  side  of  the  brass  prism  is  downwards,  and 
slides  in  a  holder  which  is  supported  by  the  sub-stage.  To 
this  brass  prism  are  attached  two  arms,  which  serve  to  support 
a  thin  screen  at  some  distance  below  the  stage.  The  screen 
is  parallel  to  that  face  of  the  glass  prism  that  receives  the  light, 
and  is  pierced  with  a  pin-hole,  which  permits  the  passage  of  a 
minute  beam  of  sunlight.  With  this  apparatus,  Col.  Dr. 
Woodward  secured  very  fine  results. 

Tolles'  Illuminating  Traverse  .Lens. — The  most 
perfect  arrangement  for  allowing  a  beam  of  light  to  reach, 
without  refraction,  a  suitably  immersed  object,  is  the  illumina- 
ting traverse  lens  of  Mr.  Tolles.  The  engraving,  Fig.  51,  and  the 
description,  which  is  in  Mr.  Tolles'  own  words,  are  from  the 
Journal  of  the  Royal  Microscopical  Society. 


OP  TSE  MICROSCOPE.  161 

The  device  is  represented  in  the  annexed  figure,  where  P  is 
the  basilar  plate  of  the  whole  traverse  system,  having  a  circular 
groove  and  track,  in  which  the  carriage,  C,  moves.  On  a  pro- 
jecting arm,  A,  of  the  carriage,  C,  are  mounted  whatever  ap- 
pliances are  to  be  used  to  modify  or  direct  the  light  upon  the 
traverse  lens,  T,  in  the  direction  of  the  object  at  the  centre  of 
the  system. 

In  the  figure  the  concave  lens,  N,  is  shown  in  position  on  the 
arm.  Thus  situated,  the  interior  convex  and  concave  surfaces 
being  of  no  effect,  the  two  exterior  plane  surfaces  of  the  traverse 


Fig.  51.— TOLLE'S  ILLUMINATING  TRAVERSE  LENS. 

system  constitute  it  a  prism,  and  every  slightest  movement  of 
this  concave  facet  lens  on  the  traverse  lens,  T,  would  would  give 
a  different  prism  to  infinite  variety.  In  this  arrangement,  the 
concave  mirror  can  be  used  in  the  ordinary  manner  and  con- 
dense light  enough  upon  the  object  for  all  ordinary  purposes. 
The  full  interior  aperture  of  a  dry  objective  would  be  reached 
at  the  very  convenient  obliquity  of  41°,  i.e.,  at  less  than  the 
critical  angle,  or  angle  of  total  internal  reflection  between 
crown-glass  and  air.  L  is  a  double-convex  condensing  lens, 
that  may  be  placed  at  about  its  principal  focal  distance  from 
the  object. 

For  a  condenser,  with  the  size  of  apparatus  as  drawn  in  the 
figure,  a  simple  lens  of  1^  inch  focus,  and  about  ten  degrees 
(10°)  of  aperture  is  convenient,  and  if  the  lens  is  movable 
along  the  arm,  A,  it  can  be  focussed  readily  on  the  object,  the 
position  being  fixed  by  inspection,  This  would  be  well  for 
parallel  rays.  If  diverging  rays  are  used,  another  lens  of  two 


162  SELECTION    AND    USE 

or  three  inches  focus,  mounted  on  the  arm,  A,  will  conveniently 
take  up  the  rays  from  the  radiant  at  the  distance  of  the  focus 
of  this  supplementary  lens. 

The  plate,  P,  is  graduated  on  its  circular  edge,  as  in  the 
figure,  to  two  degrees,  and  the  arm,  A,  has  a  swing  of.  seventy 
degrees  of  arc  each  way  from  the  axis  of  the  microscope.  An 
index-line  is  marked  on  the  bevelled  edge  of  the  carriage  10° 
from  the  axis  of  the  condenser,  which  must  be  added  to  or  sub- 
tracted from  the  real  obliquity  of  the  illuminating  rays. 

It  is  obvious  that  any  observation  made  and  duly  recorded  as 
to  its  conditions,  as  of  obliquity  of  incidence  of  illuminating 
pencil  or  ray,  form  of  the  pencil  or  beam,  focal  length  and  dis- 
tance of  the  condenser,  such  observation  could  be  successfully 
repeated.  The  record  of  the  obliquity  of  the  most  oblique  rays 
reaching  the  object  directly  and  giving  view  of  it  at  the  eye- 
piece with  luminous  field,  would  express  the  "balsam"  aper- 
ture, or  more  correctly,  the  half  interior  aperture  of  the  objec- 
tive when  the  front  lens  of  the  objective  and  the  traverse  system 
are  of  glass  of  similar  refraction. 

Having  thus  the  "balsam"  angle,  we  readily  calculate  or 
learn  the  corresponding  angle  for  glycerine  or  water,  or  any 
medium  of  which  we  have  the  index  of  refraction.  A  corres- 
ponding notation,  perhaps  for  air,  might  be  engraved  in  juxta- 
position on  the  basilar  plate. 

The  Spot  Lens.— This  is  a  plano-convex  lens  of  very  high 
curvature  (it  is  generally  hemispherical),  so  mounted  that  its 
distance  from  the  object  may  be  adjusted  in  such  a  way  that 
the  rays  which  pass  through  it,  may  be  brought  to  a  focus 
on  the  object.  The  central  jrays  are  stopped  out  by  means  of  a 
black  spot  (hence  the  name),  so  that  the  object  is  illuminated 
wholly  by  rays  which  are  of  too  great  obliquity  to  enter  the 
object-glass,  except  when  their  direction  is  changed  by  the 
object.  The  latter,  therefore,  appears  brilliantly  illuminated 
on  a  dark  ground,  and  in  many  cases  features  which  could  not 
otherwise  be  seen  are  shown  very  distinctly. 

The  larger  diatoms,  insect  scales,  and  similar  objects  are 
peculiarly  suited  to  this  method  of  illumination,  and  some  of 
them  are  very  beautiful  as  "'show  "  objects. 


OF    THE    MICKOSCOPE. 


163 


The  Parabolic  Illuminator.— This  is  an  instrument 
intended  to  accomplish  the  same  end 
as  the  spot  lens,  but  in  a  far  more 
efficient  manner.  It  consists  of  a 
block  of  glass,  the  outer  form  of  which 
is  a  parabola  with  a  cup-shaped  de- 
pression cut  in  the  upper  end.  It  is 
mounted  in  a  brass  fitting,  which 
slides  up  and  down  in  the  sub-stage 
of  the  microscope,  and  thus  may  be 
readily  adjusted,  so  as  to  throw  the 
light  properly  upon  the  object.  The 

results  obtained  by  means  of  the  parabolic  illuminator  are 

wonderfully  beautiful. 

Polarized  .Light.— The  micro  polariscope  consists  of  two 
distinct  parts,  a  polarizer  and  an  analyzer,  each  of  which  is 
now  generally  formed  of  a  Nichol  prism  properly  mounted.  A 


Fig.  52. — THE  PABABOLIC 
ILLUMINATOR. 


Fig.  53. — POLARIZER. 


Fig.  54. — ANALYZES. 


common  method  of  mounting  the  polarizer  is  shown  in  Fig.  53. 
As  there  shown,  the  lower  ring  is  intended  to  slip  into  the  ring 
of  the  sub-stage,  the  rack  and  pinion  of  which  enables  us  to 
place  the  end  of  the  prism  at  a  proper  distance  from  the  object. 
When  the  microscope  is  not  provided  with  a  sub-stage,  the 
polarizer  is  turned  upside  down,  and  the  brass  fitting  slipped 
into  a  ring,  which  is  attached  to  the  under  side  of  the  stage. 
The  milled  ring,  which  is  shown  uppermost  in  the  figure, 
enables  us  to  rotate  the  prism  in  both  cases. 

The  analyzer  may  be  arranged  in  either  one  of  two  ways.     It 
maybe  slipped  over  the  eye- piece,  or  it  may  be  mounted  in  a 


164  SELECTION  AND   TJSE 

* 

brass  tube,  the  upper  end  of  which  has  an  external  Society 
screw  that  attaches  it  to  the  body,  while  the  lower  end  of  the 
tube  has  an  internal  Society  screw  for  receiving  the  objective. 
Fig.  54  shows  the  latter  arrangement. 

Polarized  light,  except  for  the  mere  beauty  of  its  effects, 
has  not  received  the  attention  that  it  deserves.  In  some  de- 
partments of  scientific  investigation,  especially  mineralogy  and 
geology,  its  use  has  afforded  very  satisfactory  and  brilliant  re- 
sults. As  regards  its  applications  to  medicine  and  physiology, 
Dr.  Frey  says:  "  The  examination  of  tissues  by  polarized 
light  has  a  high  scientific  value,  as,  by  this  means,  molecular 
relations  become  evident,  which  by  investigation  with  ordinary 
light,  remain  entirely  concealed.  The  interpretation  of  what 
is  seen,  is  in  many  cases  difficult,  and  generally  lies  within  the 
province  of  optics,  with  which  the  medical  observer  is  usually 
but  little  familiar." 

To  detail  the  method  of  using  it,  and  the  special  features 
which  it  discloses,  would,  however,  far  transcend  the  limits  of 
this  work,  and  we  must,  therefore,  refer  the  reader  to  some 
special  treatise  on  the  subject. 

HOW  TO  USE  THE  MICKOSCOPE. 

The  remarks  which  we  are  now  about  to  offer,  are  intended 
for  the  merest  beginners — for  those,  in  fact,  who  have  never 
used  a  microscope  at  all;  and  therefore  they  may,  perhaps,  to 
some,  appear  childishly  simple.  And  yet  we  have  seen  not 
only  teachers,  but  professors  in  colleges,  y/ho  might  have  de- 
rived some  benefit  even  from  these  simple  hints.  We  remem- 
ber on  one  occasion  seeing  a  professor  of  botany  attempt  to 
examine  a  minute  plant  with  a  common  pocket  magnifier  with 
three  lenses.  In  the  first  place  he  turned  the  instrument  wrong 
side  up,  so  that,  although  he  could  see  through  it,  the  results 
attained  were  very  inferior  to  what  they  would  have  been  if  the 
instrument  had  been  properly  used;  in  the  second  place  he 
wore  his  hat  in  such  a  way  as  to  cut  off  nearly  all  the  light,  and 
in  the  third  place  he  did  not  know  how  to  hold  his  hands  so  as 
to  obtain  the  requisite  degree  of  steadiness.  If  he  had  given 
a  few  minutes  thought  to  the  subject,  he  could  no  doubt  have 


OF    THE    MICROSCOPE.  165 

corrected  his  bad  methods,  but  then  he  evidently  had  never 
considered  it  worthy  of  earnest  thought,  although  it  formed 
the  very  foundation  of  his  powers  of  observation. 

Simple  Hand  Magnifiers.— These  are  perhaps  the  most 
important  of  all  optical  instruments,  and  yet  we  rarely  find  a 
person  who  can  use  them  efficiently.  There  are  but  three 
points  that  require  attention,  viz :  The  proper  position  of  the 
magnifier  itself,  the  perfection  of  the  illumination,  and  the 
steadiness  with  which  the  instrument  is  held  at  the  exact  focal 
distance  from  the  object.  Many  magnifiers  are  so  constructed 
that  it  is  impossible  to  place  them  in  a  wrong  position;  the 
side  which  should  go  next  the  eye,  and  the  side  which  should 
go  next  the  object  are  so  well  marked  that  no  mistake  can  be 
made.  The  greatest  liability  to  error  exists  where  two  or  three 
lenses  of  different  powers  are  fixed  in  the  same  frame  and  used 
together.  This  forms  one  of  the  most  common  and  useful  of 
our  magnifiers,  and  the  rule  is  always  to  place  the  lens  of  great- 
est power  nearest  to  the  object.  Plano-convex  lenses  should  be 
placed  with  the  plane  or  flat  side  next  the  object. 

Hand  magnifiers  are,  in  the  majority  of  cases,  used  for  ex- 
amining opaque  objects,  and  one  of  the  most  important  con- 
ditions for  perfect  vision  is  that  the  object  be  well  illuminated. 
First  of  all,  then,  see  that  the  light  falls  full  and  direct  on  the 
object;  then  place  the  magnifier  as  nearly  in  focus  as  can  be 
done  without  actually  looking  through  the  lens,  and,  after  this, 
approach  the  eye  to  the  magnifier.  The  errors  most  commonly 
committed  are:  Turning  the  object  away  from  the  light;  cut- 
ting off  the  light  by  the  projecting  brim  of  a  hat  or  cap; 
shading  the  object  by  the  hand  or  the  lens  itself;  attempting  to 
examine  an  object  in  a  room  that  is  not  sufficiently  lighted. 

Having  secured  a  proper  position  for  the  magnifier  and  a 
good  illumination,  the  next  step  is  to  devise  some  means  for 
holdiog  the  lens  steadily  in  focus  during  the  examination.  This 
is  most  readily  effected  by  resting  the  hand  that  holds  the  lens 
upon  the  hand  that  holds  the  object.  Lens  and  object  then 
move  together,  and  the  focussing  remains  unchanged. 

Compound  Microscopes.— We  presume  that  the  instru- 
ment in  hand  is  a  very  simple  one,  and  that  tl^  magnifying 


166  SELECTION   AND    USE 

power  to  be  used  is  not  very  great.  No  person  should  attempt 
to  use  high  powers  and  complicated  instruments  until  he  has 
served  an  apprenticeship  by  using  a  microscope  of  simple  con- 
struction, and  objectives  of  considerable  length  of  focus. 

Let  the  beginner  commence  by  examining  some  transparent 
object  already  mounted.  To  do  this,  set  the  microscope 
on  a  firm  table,  in  front  of  a  window  by  day,  or  before 
a  lamp  at  night.  Direct  sunlight  is  to  be  avoided,  the  light 
from  a  white  cloud  being  usually  preferred  to  any  other 
source  of  illumination.  At  night  use  a  gaslight  that  does 
not  flicker,  such  as  an  argand  burner,  or  a  good  kerosene 
lamp,  the  German  student  lamp  being  very  well  suited  to  this 
purpose.  Good  results  may,  however,  be  obtained  from  any  of 
the  ordinary  lamps,  especially  those  with  a  circular  wick,  which 
are  now  so  common.  Very  fair  work  may  also  be  done  by 
means  of  a  good  candle.  This  subject  has,  however,  already 
been  discussed  at  greater  length  in  another  section. 

If  the  microscope  be  a  cheap  French  one,  the  objectives  will 
be  found  attached  to  the  body,  there  seldom  being  any  special 
provision  made  for  holding  them.  But  with  all  American  and 
English  microscopes,  and  the  better  class  of  instruments  from 
the  continent  of  Europe,  special  boxes  are  provided  for  holding 
the  objectives.  These  boxes  are  usually  made  of  brass,  and 
are  indispensable  to  the  microscopist  that  endeavors  to  take 
good  care  of  his  instrument.  Where  the  objective  is  kept  in  a 
separate  box,  the  body  of  the  microscope  must  be  raised  to  a 
sufficient  height,  and  the  objective  screwed  into  its  place.  In 
doing  this  be  very  careful  not  to  let  the  objective  fall  and  strike 
against  the  stage.  We  have  seen  more  than  one  good  lens 
spoiled  through  such  an  accident. 

When  the  objective  has  been  properly  secured  in  its  place, 
move  the  body  of  the  microscope  up  or  down  until  the  front 
lens — that  is,  the  lens  which  is  nearest  to  the  object,  is  about  a 
quarter  of  an  inch  above  the  stage.  Then  turn  the  mirror 
until  the  light  from  the  window  or  lamp  is  reflected  through 
the  microscope,  so  that  when  looking  through  it  a  bright  cir- 
cle of  light  is  seen. 

Place  on  the  stage  some  mounted  object  of  large  size,  such 
as  a  fly's  wing  or  section  of  wood.  If  a  low  power  objective  be 


OP    THE    MICROSCOPE.  167 

used,  say  one  magnifying  less  than  100  diameters,  move  ths 
body  of  the  microscope  up,  so  as  to  increase  the  distance  be- 
tween the  objective  and  the  object.  At  the  same  time  keep 
your  eye  at  the  eye-piece  and  wateh  closely.  At  a  certain  point 
the  object  will  be  seen  with  great  distinctness;  it  is  then  in 
focus,  and  is  ready  for  examination.  Always  begin  with  low 
powers.  One  of  the  greatest  risks  that  the  beginner  runs  is 
that  of  breaking  the  objective  by  forcing  it  down  on  the  object. 
To  avoid  this  with  high  powers,  bring  the  objective  down  al- 
most into  contact  with  the  slide;  when  doing  this  do  not  look 
through  the  microscope,  but  watch  the  objective,  and  stop 
whenever  it  is  sufficiently  near  the  object.  Then  apply  the  eye 
to  the  eye-piece,  slowly  raise  the  body,  and  watch  for  the  com- 
ing of  the  object  into  focus.  This  is  the  only  safe  method  with 
high  powers. 

Before  attempting  to  place  an  object  on  the  stage,  or  to  re- 
move one  from  it,  see  that  the  objective  is  raised  at  least  half 
an  inch  above  the  stage.  By  attempting  to  introduce  a  new 
slide  without  raising  the  objective,  when  using  high  powers, 
you  run  great  risk  of  injuring  both  the  object  and  the  objec- 
tive. And  in  removing  objects  from  the  stage,  never  lift  them 
up;  always  slide  them  off.  In  lifting  them  up,  great  danger  is 
incurred  of  bringing  them  into  contact  with  the  objective,  and 
thus  doing  irreparable  injury.  Sliding  entirely  prevents  this. 

Where  the  microscope  is  not  provided  with  mechanical 
means  for  adjusting  the  focus,  such  as  a  screw  or  rack  and 
pinion,  a  great  deal  may  be  accomplished  by  special  methods 
of  manipulation.  Thus  if,  instead  of  pushing  the  body  directly 
through  the  collar,  it  be  moved  with  a  slightly  twisting  motion, 
the  focus  may  be  adjusted  with  considerable  delicacy,  and  when 
the  microscope  is  not  provided  with  a  fine  movement,  a  great 
deal  may  be  done  by  means  of  a  slight  pressure  of  the  fingers 
on  the  stage.  Few  stages  are  sufficiently  rigid  to  resist  even 
the  slightest  pressure. 

The  chief  points  which  the  beginner  should  endeavor  to 
study  are  the  variations  which  are  made  in  the  appearance  of 
the  object  by  means  cf  sligbt  changes  in  the  focussing  and  the 
mode  of  illumination.  Experienced  microscopists  constantly 
keep  their  fingers  on  the  fine  adjustment  of  the  microscope. 


168  SELECTION   AND    USE 

and  watch  the  different  appearances  which  are  produced  by 
a  change  in  the  mode  of  illumination.  Swinging  the  mirror  to 
one  side,  so  as  to  send  the  light  through  the  object  in  an 
oblique  direction,  or,  where  the  mirror  cannot  be  turned  to 
one  side,  merely  turning  it  on  the  trunnions  which  support  it, 
will  often  produce  most  important  effects. 

From  what  has  previously  been  said  in  regard  to  the  neces- 
sity for  clear  and  brilliant  sources  of  illumination,  the  young 
microscopist  may,  perhaps,  be  led  to  suppose  that  the  field  of 
view  cannot  be  too  brilliantly  illuminated.  Such,  however,  is 
far  from  being  the  case.  With  ordinary  powers  (those  below 
500  diameters)  it  is  almost  always  necessary  to  moderate  the 
light,  evien  of  a  flat- wicked  lamp,  and  still  more  that  of  a 
students'  lamp.  The  finer  details  of  an  object  cannot  possibly 
be  made  out  if  the  illumination  be  too  strong;  they  are 
"  drowned  out,"  and  the  whole  object  becomes  what  artists  and 
engravers  call  flat.  The  light  may  be  regulated  by  the  dia- 
phragm which  has  been  previously  described.  Where  the  mi- 
croscope is  not  furnished  with  a  diaphragm,  increasing  the  dis- 
tance of  the  lamp  from  the  instrument  is  the  best  mode  of 
lessening  the  intensity  of  the  light. 

Very  bright  light  is  exceedingly  trying  to  the  eyes,  and 
therefore  the  student  will  find  it  advantageous  to  use  lights  of 
moderate  intensity,  and  to  increase  their  efficiency  in  every 
possible  way.  This  may  be  done  to  a  very  great  extent  by 
judicious  management — chiefly  by  excluding  from  the  eye  all 
unnecessary  light.  In  a  room  very  brilliantly  lighted  with  a 
number  of  powerful  argand  burners,  it  would  be  impossible  to 
secure  the  proper  illumination  of  a  microscopic  object  by 
means  of  a  candle,  for  the  eye,  accustomed  to  the  bright  light, 
would  fail  to  be  impressed  by  the  weaker  one.  Extinguish  the 
bright  lights,  give  the  eye  a  short  time  for  rest,  and  the  candle 
will  answer  very  well.  The  principle  thus  illustrated  finds  a 
practical  application  in  the  use  of  pasteboard  shades  surround- 
ing the  eye-piece,  and  excluding  from  the  eye  all  light  except 
that  which  passes  through  the  microscope.  Such  a  shade  is 
easily  made  and  adapted  to  any  microscope,  and  is  of  great 
service.  We  have  also  in  our  own  practice  carried  out  the 
game  principle  by  means  of  extra  diaphragms  to  our  eye- 


OF  THE  MICROSCOPE.  169 

pieces,  thus  cutting  off  all  the  light  which  passes  through  the 
microscope,  except  that  which  actually  serves  to  illuminate  the 
object. 

It  will  also  be  found  of  great  importance  to  secure  perfect 
purity  in  the  special  illumination  employed.  Thus,  if  we  are 
examining  an  object  by  transmitted  light,  it  always  detracts 
from  the  clearness  and  beauty  of  the  image  if  light  is  reflected 
from  its  surface.  It  is,  therefore,  of  advantage  to  shade  the 
object  by  means  of  a  small  tin,  brass  or  pasteboard  shade,  at- 
tached to  the  stage  so  as  to  prevent  any  light  from  the  lamp 
from  falling  on  the  object. 

A  difficulty  which  frequently  occurs  to  young  microscopists 
consists  in  the  almost  impossibility  of  securing  a  field  of  view 
equally  illuminated  in  all  par!  s.  Assuming  that  the  mirror  is 
in  proper  position,  and  that  there  is  nothing  to  shade  any  part, 
it  will  in  general  be  found  that  the  difficulty  arises  from  the 
fact  that  the  mirror  throws  images  of  the  lamp,  etc.,  upon  the 
object.  Sometimes  this  is  very  distinctly  seen;  the  shape  of 
the  flame  can  be  clearly  distinguished,  and  the  metal  portions 
of  the  lamp  appear  as  dark  shades.  The  cause  is  that  the  lamp 
is  at  the  exact  distance  at  which  the  mirror  forms  an  image  of 
it  on  the  upper  surface  of  the  slide,  just  as  a  lens,  held  in 
front  of  a  white  wall,  will  throw  an  inverted  image  of  a  lamp 
or  candle  on  the  wall,  provided  the  relative  distances  of  the 
wall,  lens  and  candle  are  properly  adjusted.  The  remedy  is 
very  simple;  move  the  lamp  either  towards  the  microscope  or 
away  from  it,  as  may  be  most  convenient. 

As  previously  stated,  the  character  of  the  illumination  af- 
forded by  a  mirror,  and  by  a  white  surface  placed  at  a  short 
distance  from  the  object,  are  appreciably  different.  A  very 
pleasant  method  of  illuminating  transparent  objects  consists  in 
the  use  of  a  plate  of  plaster  of  paris.  Its  whiteness  is  probably 
as  pure  as  that  of  any  other  substance,  and  it  is  easily  procured. 
The  plate  we  use  was  cast  in  the  cover  of  an  old  tin  box,  half  an 
inch  deep  and  three  inches  in  diameter.  A  flat  surface  was 
secured  by  casting  it  upon  a  board.  If  cast  on  glass  or  metal, 
the  surface  is  glazed  and  shiny,  which  is  bad.  Instead  of  plas- 
ter, fine  white  paper  or  cardboard  may  be  used.  Such  surfaces 
must  not  be  glazed,  and  they  should  be  kept  scrupulously  clean. 


170  SELECTION  AND    USE 

The  light  is  also  sometimes  modified  by  passing  it  through 
ground  or  colored  glass — blue  being  a  special  favorite.  Such 
light-modifiers,  as  they  are  called,  produce  a  pleasant  and 
equable  illumination,  which  is  a  great  relief  to  the  eyes,  but, 
except  for  the  resolution  of  finely  lined  objects,  we  have  not 
found  them  otherwise  of  any  special  advantage.  When  it  is 
desired  to  obtain  the  greatest  resolving  power  that  a  lens  is 
capable  of  affording,  the  blue  cell,  as  it  is  called,  is  probably  the 
most  efficient  accessory.  This  is  simply  a  glass  tank,  somewhat 
like  a  zoophyte  trough,  filled  with  a  solution  of  oxide  of  copper 
in  liquor  ammoniEe.  The  solution  is  prepared  by  adding 
liquor  ammonise  to  a  saturated  solution  of  sulphate  of  cop- 
per, until  the  precipitate  which  is  first  formed  is  re-dis- 
solved. The  intensity  of  the  blue  may  be  regulated,  either 
by  diluting  the  solution,  or  by  varying  the  thickness  of  the 
layer  of  liquid. 

When  it  is  desired  to  examine  any  thing  by  light  reflected  from 
it,  instead  of  light  transmitted  through  it,  the  object  should  be 
placed  before  a  dead-black  surface,  such  as  the  dark  part  of  the 
diaphragm,  or  a  blackened  card,  and  at  such  a  distance  from  it 
that  the  surface  of  the  background  is  not  in  focus.  Then, 
place  the  condensing  lens  in  relation  to  the  lamp,  so  that  a 
bright  spot  of  light  will  fall  on  the  object,  and  on  bringing  it 
into  focus  it  will  be  clearly  seen.  Low  powers  only  can  be 
satisfactorily  used  for  the  examination  of  opaque  objects  by 
beginners. 

The  beginner  should  commence  with  the  simplest  mounted 
objects,  and  afterwards,  when  a  little  skill  in  the  manipulation 
of  the  instrument  has  been  acquired,  he  should  proceed  to  the 
examination  of  such  simple  unmounted  objects  as  are  easily 
prepared.  The  latter  course  will  prove  altogether  the  most 
valuable  and  instructive,  for  he  who  confines  himself  to  the  ex- 
amination of  mounted  objects  only  can  never  hope  to  become  a 
microscopist.  After  a  time,  when  a  little  skill  has  been 
acquired  in  the  preparation  of  objects,  the  student  may  proceed 
to  preserve  and  mount  them.  Most  young  people  try  to  mount 
before  they  have  learned  to  prepare  objects,  and  the  conse- 
quence is  that  they  soon  find  themselves  in  possession  of  a 
large  collection  of  very  poor  slides. 


OF  THE  MICROSCOPE.  171 

On  the  Use  of  Objectives  of  Large  ^Apertures.— 

When  the  first  edition  of  this  book  was  issued,  wide-angled  ob- 
jectives were  far  from  common.  The  "Battle  of  the  Object- 
Glasses  "  was  at  its  height,  and  objectives  capable  of  resolving 
the  Amphipleura  pettucida,  or  Nobert's  nineteenth  band,  by  sim- 
ple lamplight,  were  comparatively  scarce.  During  the  interven- 
ing years  the  opticians  have  been  hard  at  work,  and  have  turned 
out  objectives  of  a  high  class  to  such  an  extent  that  almost 
every  microscopical  society  numbers  amongst  its  members  those 
who  have  glasses  of  high  balsam  apertures-.  It  is  an  unfortu- 
nate fact,  however,  that  thus  far  the  text-books  are  entirely  in- 
nocent of  any  directions  for  using  these  glasses.  We  have  now 
before  us  a  treatise  of  over  400  pages,  which  left  the  author's 
hands  as  late  as  the  middle  of  1880,  and  which  does  not  contain 
a  single  direction  for  the  use  of  the  cover  correction  !  As  a 
justification  of  such  omission,  it  has  been  alleged  that  objec- 
tives of  large  aperture  require  no  more  care  and  skill  than 
others.  From  this  position  we  most  emphatically  dissent,  and 
if  evidence  were  wanting  we  could  cite  the  case  of  a  prominent 
officer  of  one  of  our  microscopical  societies,  and  one  who  claims 
to  be  an  expert  in  the  use  of  the  microscope,  who  thought  it  a 
great  feat  to  show  No.  18  of  the  Probe  Platte  with  a  £  objective, 
which  undoubtedly  was  capable  of  resolving  No.  20  handsomely 
if  properly  handled  1  It  is  now  many  years  since  we  saw  Mr. 
Tolles  resolve  No.  20  with  a  quarter  inch  objective  and  a  com- 
mon hand  lamp  burning  kerosene  oil. 

Instruction  in  the  proper  methods  of  handling  first-class 
objectives  is  best  obtained  from  a  living  teacher.  It  will  be 
found  one  of  the  most  difficult  things  to  learn  from  a  book. 
One  reason  for  this  is  that  until  he  has  become  expert,  or  has 
seen  the  objects  in  the  hands  of  some  one  who  is  expert,  the 
student  does  not  know  what  appearance  to  look  for.  The  con- 
sequence is,  that  he  is  all  the  time  working  in  the  dark.  But 
after  he  has  seen  the  Amphipleura  or  the  Saxonica  well  shown 
by  some  one  who  knows  how  to  handle  a  good  objective,  ho 
has  a  standard  of  excellence  to  go  by,  and  it  will  be  very  strange 
if,  after  a  few  trials,  he  does  not  surpass  the  work  of  his  teacher. 
Then,  as  soon  as  he  has  learned  to  bring  out  what  he  knows  to 


172  SELECTION   AND   USE 

be  the  best  results  on  difficult  diatoms,  lie  knows  when  his 
lenses  are  doing  good  work  in  his  hands,  and,  so  far  as  his  in- 
strument is  concerned,  he  feels  confident  that  he  can  apply  it  to 
any  class  of  objects  and  get  views  that  are  trustworthy. 

To  get  the  best  results  from  modern  objectives  of  wide  angle 
of  aperture,  there  are  two  things  that  must  be  carefully  at- 
tended to — the  illumination  of  the  object,  and  the  adjustment 
of  the  correction  for  the  thickness  of  cover-glass.  Of  delicacy 
in  focussing,  which,  by  the  way,  is  a  most  important  point,  it 
is  unnecessary  to  speak. 

As  regards  illumination,  the  three  great  points  that  must  be 
secured  are  purity,  intensity,  and  suitability.  By  purity  we 
mean  that  the  light  must  be  wholly  of  a  certain  degree  of 
obliquity,  for  these  wide-angle  lenses  gather  in  so  many  rays, 
that  a  diffused  light,  which  does  not  affect  low-angled  objec- 
tives, greatly  injures  the  working  qualities  of  those  of  high 
angle.  Thus,  for  example,  if  we  are  working  by  central  light, 
it  is  necessary  to  shut  out  all  the  direct  rays  from  the  lamp, 
which  would  enter  at  every  sort  of  angle,  and  produce  con- 
fusion. A  narrow  pencil,  produced  by  sending  a  beam  of 
parallel  rays  of  intense  light  through  a  small  hole  placed  some 
distance  below  the  stage,  will  give  central  illumination,  which 
will  give  very  different  results  from  that  obtained  by  a  dull, 
diffused  light,  such  as  may  be  obtained  from  the  flat  side  of  the 
wick  of  a  lamp,  falling  on  a  large  mirror  and  reflected  upward. 

When  oblique  illumination  is  used,  great  care  should  be 
taken  to  prevent  rays  of  different  degrees  of  obliquity  from 
falling  on  the  object.  A  large,  concave  mirror  fills  a  large 
portion  of  the  arc  through  which  it  swings,  and  its  upper  and 
lower  edges  reflect  rays  which  fall  upon  the  object  with  very 
different  degrees  of  obliquity.  Where  very 'oblique  rays  are 
used,  but  a  small  proportion  of  them  enter  the  glass  slide, 
unless  they  are  "  guided  "  through  by  some  such  contrivance 
as  the  Eeflex  Illuminator,  Tolles'  Traverse  Lens,  etc.  Kays  of 
less  obliquity  enter  much  more  freely,  and  although  less  in 
quantity  than  the  others  in  the  first  place,  they  drown  them 
out.  In  the  exclusion  of  these  rays  consists  in  a  great  degree 
the  value  of  many  of  the  "illuminators"  in  common  use,  and 
the  same  effect  may  be  secured  to  a  considerable  extent  by 


OF  THE   MICKOSCO_  173 

means  of  a  simple  screen.  This  fact  lias  been  made  very  ap- 
parent by  Prof.  J.  E.  Smith,  who,  by  the  nse  of  a  simple 
"  oblique  diaphragm,"  as  it  is  called,  has  secured  results  which 
were  previously  supposed  to  demand  much  more  complicated 
and  expensive  arrangements.  This  oblique  diaphragm,  or 
screen,  consists  of  a  plate  of  very  thin  metal  secured  to  the 
under  surface  of  the  stage.  The  stage  then  forms  the  upper 
surface  of  a  >,  and  the  metal  plate  the  lower  one.  The  angle 
may  be  adjusted  at  will  by  simply  bending  the  plate,  and  it  is 
evident  that  all  rays  from  below  will  be  entirely  excluded. 
A  piece  of  the  thin  iron  known  as  ferrotype  ptate  is  the  cheap- 
est and  best  material  for  this  purpose.  Its  surface  should  be 
dead  black. 

The  same  result  is  obtained  by  the  use  of  a  conical  dia- 
phragm fitted  to  the  sub-stage. 

When  such  objects  as  difficult  tests  are  viewed  by  oblique 
light,  it  will  be  found  that  there  is  a  certain  angle  of  illumina- 
tion at  which  the  objective  will  generally  perform  best,  and 
this  must  be  found  by  careful  experiment.  The  higher  the 
angle  of  aperture  of  the  objective,  the  greater  may  be  the  angle 
at  which  the  object  is  illuminated,  but  it  will  be  found  that 
many  objectives  fail  to  work  up  to  the  full  angle  claimed  for 
them  by  their  makers. 

Such  are  the  general  principles  to  which  the  student  must 
pay  attention  in  regard  to  the  matter  of  illumination,  but  in  ad- 
dition there  are  numerous  minor  details,  a  knowledge  and  appre- 
ciation of  which  can  only  be  acquired  by  practice.  The  finer 
objectives  are  so  sensitive  to  the  slightest  changes,  that  the 
least  movement  of  the  mirror  or  lamp  influences  the  result  in 
a  very  marked  degree. 

The  other  important  point  to  be  attended  to  is  the  cover- 
correction.  This  will  be  found  to  demand  great  patience  and 
attention.  The  older  authorities  give  fixed  rules  for  regulating 
the  cover-adjustment,  but  as  it  unfortunately  happens  that  this 
adjustment  varies  not  only  with  the  thickness  of  the  cover« 
glass,  but  with  the  depth  to  which  the  object  is  sunk  in  the 
mounting  medium  (and  this  is  not  always  the  same),  and  the 
angle  of  illumination,  it  will  be  seen  that  each  object  requires 
special  attention  in  regard  to  these  points.  In  addition  to  this, 


174  SELECTION  AND  USE 

it  will  be  found  that  each  objective  has  its  own  special  charac- 
teristics, which  must  be  carefully  studied  by  the  owner  if  he 
would  command  success. 

The  difficulty  of  giving  any  rules  which  will  enable  the 
student  to  put  the  "finer  touches"  on  this  kind  of  work,  is  well 
set  forth  by  Dr.  Blackham  in  a  recent  article,*  from  which  we 
quote  as  follows  : 

"It  will  probably  be  expected  that  something  should  be  said 
here  in  reference  to  the  adjustment  of  the  objective  for  dif- 
ferent cover-glasses,  etc.,  by  means  of  .the  screw  collar,  but  on 
this  point,  unfortunately,  but  little  can  be  said,  though,  of 
course,  it  is  a  most  important  one,  and-  the  better  the  objective, 
and  the  wider  its  angle,  the  more  important  is  accurate  adjust- 
ment. Every  wide-angled  immersion  objective  that  is  worth 
having,  is  a  separate  work  of  art,  and,  as  such,  has  an  individ- 
uality with  which  the  worker  must  become  acquainted,  and 
which  he  must  learn  to  turn  to  his  advantage. 

"  None  of  these  lenses  which  I  have  seen  are  perfectly  achro- 
matic, and  each  has  a  special  wave  length  at  which  it  does  its 
best.  In  Tolles',  and  Bausch  &  Lomb's,  and  I  believe  in 
Spencer's  lenses,  this  is  between  the  blue  and  the  green,  but 
the  exact  shade  differs  with  different  lenses,  and  must  be  found 
by  experiment.  My  plan  is  to  adjust  roughly  by  means  of  the 
tint  of  the  field,  then  to  bring  an  object  into  the  field  (if  we 
are  at  work  on  the  Probe  Platte,  one  of  the  easier  diatoms,  say 
Pleurosigma  angulalum],  and  focus  on  it  and  arrange  the  illu- 
mination as  accurately  as  possible,  and  then  with  the  finger  and 
thumb  of  the  left  hand,  turn  the  correction  collar  of  the  objec- 
tive backwards  and  forwards,  keeping  the  object  in  focus  all 
the  time  by  means  of  the  forefinger  of  the  right  hand  on  the 
milled  head  of  the  fine  adjustment,  until  the  best  effect  is  ob- 
tained. An  occasional  slight  change  in  the  position  of  the  mir- 
ror is  often  needed. 

"In  all  these  manipulations,  deliberation  and  care  are 
needed,  and  the  patience  of  the  beginner  will  often  be  sorely 
taxed,  but  let  him  remember  that  nothing  worth  having,  can  be 
gotten  without  trouble." 

* American  Journal  of  Microscopy  for  February,  1880. 


OF    THE    MICROSCOPE.  175 

Care  of  tlie  Microscope.  —A  microscope,  when  not  in 
use,  should  always  be  kept  well  covered,  either  in  its  case  or 
under  a  suitable  cover.  There  is  110  more  convenient  mode  of 
keeping  a  microscope  than  to  stand  it  upon  a  cloth  mat,  and 
cover  it  with  a  glass  shade.  It  is  thus  kept  free  from  dust  and 
vapors,  and  is  always  ready  for  use;  but  when  it  is  kept  in  its 
case,  and  especially  if  it  has  to  be  screwed  together,  interesting, 
valuable,  or  even  important  objects,  will  often  fail  to  be  ex- 
amined, simply  because  too  much  time  and  labor  are  necessary 
to  prepare  for  the  operation. 

A  good  microscope  should  be  so  carefully  protected,  that  it 
shall  rarely  require  to  be  cleaned  or  dusted,  as  this  wears  off 
the  lacquer,  and  exposes  the  metal,  which,  when  thus  uncov- 
ered, soon  begins  to  tarnish.  When  dusting  or  cleaning  becomes 
absolutely  necessary,  chamois  leather,  or  a  very  fine  old  linen 
or  silk  handkerchief  is  most  suitable.  Never  use  coarse  cloths, 
or  those  that  have  been  lying  about  exposed  to  dust  and  dirt. 

The  lenses  should  be  kept  in  their  boxes  when  not  in  use, 
and  when  they  are  attached  to  the  microscope,  great  care 
should  be  taken  to  keep  them  from  coming  into  contact  with 
liquids.  In  order  to  prevent  the  latter  accident  as  far  as  pos- 
sible, never  examine  liquids  unless  when  they  are  covered  with 
thin  glass.  In  the.  pursuit  of  micro-chemical  studies,  the 
microscopist  has  frequently  to  deal  with  liquids  that  corrode 
metals,  and  even  glass.  In  well-appointed  laboratories  inverted 
microscopes  are  used  in  such  cases,  but  with  ordinary  instru- 
ments, special  means  must  be  employed.  The  object  should  be 
laid  on  a  large  piece  of  thin  plate  glass,  and  the  brass  work  of 
the  objective  should  be  coated  with  oil.  The  rest  of  the  metal 
work  may  be  protected  with  oiled  silk  or  thin  india-rubber. 

When  liquids  which  corrode  glass  are  used,  the  front  of  the 
objective  should  be  protected  by  means  of  a  very  thin  leaf  of  the 
best  mica,  which  may  be  attached  either  by  glycerine  or  balsam. 

These,  however,  are  exceptional  precautions.  In  ordinary 
work  it  is  sufficient  to  see  that  the  lenses  and  metal  work  are 
kept  free  from  stains  and  finger  marks. 

Never  touch  with  the  fingers  the  surface  of  any  lenses,  either 
eye-pieces  or  objectives,  as  this  will  be  certain  to  soil  them.  Use 
raoft  camel-hair  brushes  to  remove  particles  of  dust,  etc.  Where 


176  SELECTION  AND    USE 

dirt  adheres  more  strongly,  use  fine  linen  slightly  moistened  with 
alcohol,  and  wipe  dry  with  very  fine  chamois  leather.  Remerr- 
ber,  that  alcohol,  if  used  profusely,  will  attack  the  lacquer  of 
the  brass- work,  and  even  dissolve  the  cement  .which  holds  the 
lenses  together.  When  objectives  are  smeared  with  balsam, 
the  best  cleansing  agent  is  said  to  be  kerosene  oil.  The  piece 
of  leather  used  for  wiping  lenses  should  be  free  from  dust,  and 
is  best  kept  in  a  small  box  by  itself,  and  used  for  nothing  else. 
It  must  be  remembered  that  the  glass  of  which  objectives  are 
made  is  easily  scratched,  being  soft  when  compared  with  parti- 
cles of  sand  and  grit;  consequently,  when  frequently  wiped  it 
soon  loses  that  exquisite  polish  upon  which  its  excellence  of 
performance  so  much  depends.  What,  then,  are  we  to  think 
of  the  directions  given  by  the  author  of  a  popular  work  on 
the  microscope,  in  which  we  are  told  to  use  a  piece  of  leather, 
slightly  impregnated  with  brick  dust ! !  No  better  method  of 
destroying  an  objective  could  possibly  be  devised.  Therefore, 
see  that  in  wiping,  the  slightest  possible  pressure  is  used,  lest 
any  particle  of  grit  should  make  a  scratch. 

The  exposed  parts  of  all  microscopes,  as  well  as  the  objectives 
and  their  cases,  are  lacquered,  to  protect  them  from  being 
soiled  by  handling,  but  the  interior  of  the  boxes  which  hold  the 
object-glasses  are  rarely  so  protected,  and  the  black  coating  of 
i he  interior  of  bodies,  draw-tubes,  etc.,  is  frequently  not  very 
firmly  attached.  Therefore,  never  touch  them  with  the  fingers. 

After  taking  an  objective  out  of  its  box,  either  screw  on  the 
cover  of  the  box,  or  place  the  latter  with  its  open  end  down,  Do 
not  stand  it  mouth  up,  so  that  it  may  catch  all  the  dust. 

When  exhibiting  the  microscope  to  others,  great  care  is  neces- 
sary to  keep  meddlesome  fingers  from  soiling  the  glasses.  Some 
people  are  never  content  when  merely  allowed  to  look  at  things: 
they  insist  upon  handling  them,  and  feeling  them.  To  the 
young  microscopist,  we  would  say  that  if  any  of  ^  our  friends  in- 
sist upon  handling  your  objectives,  eye-pieces,  etc.,  put  up  the 
instrument  and  pack  it  away.  A  microscope  carefully  used  is 
as  good  after  fifty  years  as  when  first  made,  but  we  have  seen 
an  instrument  suffer  more  injury  in  half  an  hour  at  the  hands 
of  a  thoughtless  and  dirty  person,  than  it  would  have  sus- 
tained in  twenty  years  in  the  hands  of  a  careful  microscopist. 


Of  THU  MICROSCOPE.  177 


COLLECTING  OBJECTS. 

Those  who  are  engaged  in  special  studies  and  researches  re- 
quire no  directions  for  collecting  objects;  but  to  those  who  use 
the  microscope  for  purposes  of  general  instruction  or  amuse- 
ment, a  few  hints  may  not  be  out  of  place.  Almost  every  text- 
book on  botany,  physiology,  mineralogy  and  kindred  subjects, 
will  not  only  indicate  a  long  list  of  objects,  but  will  give  di- 
rections for  procuring  them.  Plants  yield  a  very  large  variety 
of  interesting  subjects.  Thus  the  cuticles  of  the  leaves  and 
flowers;  cellular  tissue  as  shown  by  dissections,  and  by  cross 
and  longitudinal  sections;  hairs,  pollen,  seeds,  etc.,  all  deserve 
careful  microscopical  examination.  Insects  furnish  an  almost 
unlimited  field,  and  their  wings,  feet,  eyes,  mouth,  scales,  spira- 
cles, hairs,  etc.,  are  all  worthy  of  careful  preparation  and  exam- 
ination. 

It  is,  however,  amongst  the  more  minute  forms  of  animal  and 
vegetable  life,  as  found  in  pools  and  running  streams,  that  the 
most  interesting  objects  are  to  be  found,  and  the  number  and  var- 
iety of  these  is  so  great  that  several  large  volumes  would  be  re- 
quired to  describe  them.  Even  the  ponderous  works  of  Ehren- 
berg  and  Pritchard  do  not  begin  to  exhaust  the  subject,  and, 
therefore,  it  will  be  obvious,  that  even  if  we  were  to  devote  the 
wnole  of  the  present  volume  to  this  department,  we  could 
but  skim  the  surface.  Thus  far  we  have  had  to  depend  chiefly 
upon  foreign  works  for  descriptions  of  these  organisms,  but  it  is 
fortunate  that  while  the  higher  classes  of  plants  and  animals 
which  inhabit  Europe,  and  are  described  in  European  works, 
are  entirely  different  from  their  congeners  on  this  continent,  the 
same  does  not  hold  true  in  regard  to  the  lower  forms.  We  have 
found  localities  which  teemed  with  the  Volvox  Globator  and 
various  species  of  Closterium,  Staurastrum,  Pediastrum,  etc. 
Hydras  are  to  be  found  in  great  abundance,  and  so  nearly  like 
the  described  European  species  that  the  beginner  will  And  it 
difficult  to  detect  the  difference.  We  have  repeatedly  found  the 
Stephanoc&ras,  Meliceria  and  other  beautiful  microscopic  objects, 
and  as  for  the  more  common  ones,  such  as  the  VorticeUi,  or 


ITS  SELECTION    AND    USE 

•wheel  animalcules  and  Entomostraca,  or  water  fleas,  they  are 
to  be  found  in  every  pool. 

Every  young  microscopist  that  is  desirous  of  pursuing  his 
studies  in  this  direction,  is  met  at  the  outset  by  two  difficulties; 
the  first  is  to  obtain  the  objects,  the  second  is  to  find  out  what 
they  are  after  he  has  got  them.  The  first  is  by  no  means  a  dif- 
ficult task,  but  the  second  will  often  puzzle  more  experienced 
students  than  those  whom  we  expect  to  be  readers  of  this  book. 
We  know  of  but  two  ways  to  accomplish  it;  one  is  the  laborious 
plan  of  searching  for  them  in  the  ' '  Micrographic  Diction- 
ary," or  the  books  of  Carpenter  or  Pritchard;  the  other  is 
to  obtain  the  desired  information  from  some  well-informed 
friend. 

The  objects  which  are  of  most  interest  to  the  microscopist  are 
not  difficult  to  obtain,  if  we  know  where  to  look  for  them,  but 
they  are  not  to  be  found  everywhere.  Many  stagnant  pools 
will  be  found  to  yield  but  a  scanty  supply,  while  others,  which, 
perhaps,  to  the  uninitiated  present  a  less  promising  appearance, 
will  yield  a  rich  harvest.  Beginners  are  very  apt  to  entertain 
the  popular  notion,  that  every  drop  of  water  teems  with  animal- 
cules, and  that  when  placed  under  the  microscope,  it  will  appear 
to  be  literally  filled  with  living  things.  This  idea  is  fostered 
by  popular  writers  who  describe  a  drop  of  water  as  a  globe  filled 
with  life,  and  by  lecturers  who  exhibit  pictures  and  enlarged  im- 
ages of  wLat  they  call  "a  drop  of  water,"  but  which  is  in  reality 
a  considerable  quantity  of  that  liquid  which  has  been  artificially 
supplied  with  inhabitants.  Clear  well  water  is  almost  free  from 
microscopic  organisms,  and  the  same  is  true  of  the  water  from 
clear  brooks,  which  flow  swiftly  over  a  pebbly  bottom.  Ordin- 
ary j  ain  water,  as  found  in  cisterns  having  free  communication 
with  the  air,  usuilly  contains  large  numbers  of  the  larvae  of 
gnats  and  mosquitoes,  and  when  exposed  to  the  light  it  is  almost 
always  rich  in  wheel  animalcules,  and  some  of  the  lower  forms 
of  vegetable  life.  The  water  supplied  to  our  cities  is  in  gen- 
eral very  rich  in  microscopic  vegetables.  Thus  in  the  Croton 
water,  which  is  comparatively  pure,  we  have  found  a  large 
number  of  very  beautiful  species,  amongst  them  the  exquisite 
Monachinus.  The  best  way  to  secure  a  supply  of  the  animal 
and  vegetable  inhabitants  of  city  water,  is  to  pass  a,  considera- 


OF  THE  MICROSCOPE.  179 

ble  quantity  of  it  through  a  filter,  the  surface   of  -which  will 
then  furnish  a  large  amount  of  valuable  matter. 

But  it  is  not  in  such  .fields  that  the  microscopist  -will  find  his 
best  hunting  grounds.  Along  the  edges  of  quiet  pools  of  clear 
water  is  the  best  place  for  the  finer  vegetable  forms,  such  as 
the  Volvox  Globator,  Closlerium,  etc.  If  the  water  is  much  con- 
taminated with  dead  animal  matter  or  with  sewage,  nothing  will 
be  found  but  the  coarser  organisms  and  animalcules,  such  as 
Paramecium.  The  same  is  true  of  small  pools  found  in  woods, 
or  very  much  shaded  with  trees,  and  filled  with  dead  leaves. 
Such  places  are,  however,  the  favorite  haunts  of  the  larvae  of 
insects,  and  also  of  frogs  and  Tritons.  The  size  of  the  pools  is 
not  of  much  consequence.  We  remember  on  one  occasion  to 
have  found  by  the  roadside  in  Centre  County,  Pennsylvania,  a 
little  pool  which  was  almost  filled  with  the  larvae  of  Tritons. 
The  gills,  which  were  beautifully  developed,  would  have  formed 
a  splendid  object  under  the  microscope,  but  when  we  returned 
next  day,  for  the  purpose  of  securing  some,  the  water  had  dried 
up,  and  the  larvae  were  all  gone. 

The  little  pools  formed  in  boggy  ground  by  the  footsteps  of 
cattle  will  often  be  found  to  contain  large  quantities  of  one  or 
t,vo  species  of  desmids  or  diatoms.  It  will  not  do  to  look  for 
fiese  objects  in  similar  pools  formed  in  ordinary  soft  land,  and 
temporarily  filled  with  rain  water.  The  ground  must  be  na- 
turally and  constantly  wet,  so  that  the  pools  are  always  kept 
filled  by  the  infiltration  of  water  from  the  surrounding  soil. 
Such  pools,  however  small,  usually  contain  a  large  number  of 
specimens,  and  it  is  in  such  places  that  one  is  most  likely  to 
find  a  supply  of  one  variety  unmixed  with  any  others. 

While  many  of  the  most  interesting  objects  will  be  found 
swimming  freely  about  in  the  water,  others  of  great  beauty  are 
always  attached  to  floating  weeds,  sticks,  etc.  We  have  gen- 
erally been  most  successful  in  discovering  specimens  of  this 
kind  when  we  have  placed  the  gathering  in  a  large  glass  jar,  and 
allowed  it  to  stand  quiet  for  some  time.  The  water  will  then 
settle,  and  the  objects  of  which  the  microscopist  is  in  search 
will  have  time  to  expand,  when  they  may  be  seen  in  a  form 
resembling  light  mould,  or  down,  attached  to  the  surfaces  of 
the  solid  matters. 


ISO  SELECTION   AND    "USE 

The  surface  of  the  mud  at  the  bottom  of  ponds  of  clear  waier, 
is  frequently  very  rich  in  microscopic  vegetable  organisms. 
These  minute  plants  seem  to  seek  the  light,  and  to  rise  through 
the  mud  which  -would  otherwise  cover  them,  so  that  by  care- 
fully scraping  the  surface  of  the  bottom,  we  are  enabled  to 
procure  them  in  large  numbers. 

It  must,  of  course,  be  borne  in  mind,  that  while  some  species 
are  found  in  fresh  water,  others  are  marine,  that  is,  they  live 
only  in  sea- water.  The  best  locations  for  finding  marine  forms 
are:  1,  the  pools  of  c  ear  water,  found  in  salt  marshes;  2, 
the  surface  of  the  mud  at  the  bottoms  of  harbors  and  quiet 
coves;  3,  the  waters  of  the  ocean  itself,  as  well  as  that  of  the 
bays  and  coves  connected  with  it. 

The  apparatus  required  for  capturing  these  various  objects, 
is  neither  bulky  nor  expensive.  For  Iarva3  and  the  larger  ani- 
malcules, the  most  useful  implement  is  a  small  net.  Ours  con- 
sists of  a  ring  of  brass  wire  (iron  wire  would  rust  and  destroy 
the  net )  about  six  inches  in  diameter,  soldered  to  a  tin  tube  or 
ferrule,  which  fits  tightly  on  the  end  of  a  walking  cane.  To 
the  ring  is  attached  a  bag  of  any  light,  gauzy  material,  which 
possesses  the  two  qualities  of  letting  water  out  rapidly,  and 
keeping  small  objects  in.  With  this  net  it  is  easy  to  ^capture 
anything  from  a  small  fish  or  a  frog  to  the  very  smallest  larva,' 
and  it  is  very  portable,  since  an  ordinary  walking  cane  forms  a 
sort  of  universal  handle  for  this  and  other  implements.  Next 
to  the  net,  we  find  the  most  useful  articles  to  be  bottles.  They 
should  be  of  clear  glass,  so  that  any  object  contained  in  them 
may  be  readily  examined  by  means  of  a  pocket  lens.  For  this 
reason  we  prefer  what  are  called  homoeopathic  phials  of  large  size 
(half  ounce  and  quarter  ounce),  and  we  generally  carry  a  tfozeii 
or  two  when  out  on  a  tramp.  A  fair  sample  of  the  contents  of 
a  small  pool  is  easily  obtained  by  gently  lowering  the  phial, 
mouth  downwards,  under  the  water,  and  bringing  it  cautiously 
to  the  place  which  is  supposed  to  be  richest  in  specimens.  The 
phial  is  then  turned  mouth  upward,  the  air  rushes  out  and  the 
objects  are  carried  into  the  bottle  by  the  force  of  the  inrushing 
current  of  water.  For  small,  shallow  pools,  the  phial  is  most 
conveniently  held  in  the  hand,  but  when  the  water  is  deep  a 
handle  is  required,  and  for  this  we  use  the  holder  shown  in 


OF    THE   MICUOSCol'E.  181 

Fig.  55,  which  is  made  to  fit  on  the  end  of  the  -walking  stick.   It 
consists  of  a  ferrule  having  a  semi-cylindrical  piece  soldered  at 
right  angles  to  it.     The  ferrule  fits  the  cane, 
and  the  bottle  is  fastened  to  the  cross  piece 
by  means  of  a  rubber  ring — the  method  of 
arranging  the  latter  being  easily  understood 
from  the  engraving.     A  dozen  or  more  bottles 
of  proper  size  may  be  taken   along,  and  they 
Fig.  55.  are  so  easily  attached  to  the  holder  that  there 

is  no  necessity  for  transferring  a  "dip"  to 
another  bottle.  The  contents  are  most  easily  carried  in  the 
bottle  in  which  they  were  first  obtained. 

When  the  water  is  too  deep  for  a  walking  cane,  a  fishing  rod 
or  any  long  pole  may  be  used,  and  where  these  prove  too  short, 
as  in  harbors,  etc. ,  a  bottle  may  be  lowered  and  raised  properly 
by  means  of  strings.  For  this  purpose  the  bottle  must  be 
heavily  loaded  with  lead  round  the  neck,  and  two  strings  must 
be  attached  to  it,  one  fastened  to  the  neck  and  the  other  to  the 
bottom.  It  is  by  the  latter  that  the  bottle  must  be  lowered, 
but  it  must  be  raised  by  the  other.  If  properly  managed  it 
will  descend  mouth  downwards,  but  the  tension  of  the  string 
attached  to  the  neck  will  invert  it,  and  when  raised  by  this  string 
it  will  bring  up  its  contents  very  perfectly. 

For  scraping  the  surface  of  the  mud  at  the  bottom  of  shallow 
pools,  we  use  the  spoon  shown  in  Fig.  56.  It  is  simply  a  ring  of 
tin  five  inches  in  diameter  and  one 
inch  deep.  The  lower  edge  is 
"  wired  "  as  the  tinsmiths  call  it,  and 
there  is  a  ferrule  soldered  to  the  side 
so  that  it  may  be  fixed  to  the  same 
cane  that  is  used  for  the  net.  Over  the  Fig.  56. 

bottom  is  stretched  a  piece  of  some 

thin  fabric,  such  as  thin  muslin,  gauze  or  tarletan,  which  is  held  in 
place  by  a  rubber  band  that  slips  over  the  wire  ring  on  the  lower 
edge  It  is  best  to  make  one  side  of  the  ring  somewhat  flat,  so 
as  to  adapt  it  better  to  the  flat  surface  of  the  mud.  When  the 
pieces  of  cloth  get  soiled,  they  are  easily  replaced,  and,  indeed, 
in  some  cases  it  is  not  a  bad  plan  to  carry  the  mud  home  in  the 
Vet  cloths,  a  dozen  or  more  of  which,  with  their  contents,  may 


182 


SELECTION  AND     TJSfc 


Fig.  57. 


be  easily  packed  in  a  tin 
box  of  small  size.     One  of 
the  boxes  used  by  school 
children  for   lunch    boxes 
answers  very  well,  but  any 
tin  box  with  a  lid  or  cover 
will  answer.     As  it  is  im- 
portant that  a  record  should 
be  kept  of  the  locality  from 
which  the  dip  was  taken, 
we   carry    a  few   slips  of 
parchment  paper,   one    of 
which  is  pinned   to   each 
cloth,  after  the  necessary 
memoranda      have      been 
written  upon  it  with  a  hard 
pencil.  On  returning  home, 
the  contents  of  each  cloth 
may   be    transferred  to  a 
separate  bottle.     This  plan 
saves  the  carrying  of  nu- 
merous   bottles,    and    the 
water  required  to  fill  them. 
An    exceedingly  coveni- 
ent    traveling    companion 
for  those  who  are  fond  of 
collecting,  is  shown  in  the 
accompanying     engraving, 
Fig.   57.     The  main  part 
forms    a    very  convenient 
walking  cane  of  ordinary 
appearance.      Lake    many 
fishing  rods,  however,  it  is 
hollow,  and  contains  a  sec- 
ond rod  by  which  it  maybe 
extended  to  twice  its  length. 
This  enables  the  user   to 
reach  the  bottom  of   any 
ordinary  pond,  and  to  reach 
as  far  as  is  necessary  from 


OF    THE    MICROSCOPE. 


183 


the  shore.  Accompanying  the  cane,  A,  are  the  hooked  knife, 
B,  and  the  ring  and  bottle,  C.  These  are  made  with  a  double 
screw,  so  that  they  may  be  attached  either  to  the  end  of  the 
cane  itself,  or  to  the  inner  rod,  and  in  this  way  we  can  have 
either  a  short  and  stout  handle,  or  a  longer  and  more  slender 
one,  as  circumstances  may  require.  The  bottle  is  made  so  as 
to  screw  into  the  brass  ring,  and  the  same  screw  enables  us  to 
fit  a  wooden  cap  on  it,  which  thus  encloses  the  contents  tightly. 
The  hook  is  made  of  fine  steel,  and  has  a  sharp  cutting  edge,  as 
seen  in  the  engraving,  so  that  it  is  easy  to  cut  off  a  piece  of 
weed,  drag  it  out  of  the  water  and  secure  it. 

Those  who  carry  such  a 
cane  do  not  attract  attention 
by  any  unusual  parapher- 
nalia, and  at  the  same  time 
they  are  at  all  times  ready  to 
secure  any  valuable  material 
that  may  present  itself.  Sev- 
eral bottles  may  be  carried  in 
the  pocket,  and  screwed  into 
the  ring  as  required. 

The  collector  who  desires 
to  make  a  thorough  examina- 
tion of  the  microscopic  flora 
and  fauna  of  any  pool  or 
stream,  must  not  rest  content 
with  infinitesimal  quantities 
<>f  material.  It  is  not  neces- 
sary, however,  to  lug  home  a 
galloii  of  water  for  the  sake 
of  the  objects  contained  in  it, 
and  so  fully  have  microscop- 
ists  been  impressed  with  this 
idea,  that  the  devices  which 
have  been  prepared  for  strain- 
ing out  the. valuable  portions 
are  almost  endless.  The  best 
and  simplest  that  we  have 
seen  is*  a  modification  of  an 


Fig.  58.— WATEB-STRAINER. 


184 


SELECTION   AND    USE 


arrangement,  designed,  we  believe,  by  Mr.  Highley,  and 
figured  in  Beale's  work  on  the  microscope.  It  consists,  as 
shown  in  the  engraving  (see  Fig.  58),  of  a  bag  or  net  of 
some  light  material,  to  the  bottom  of  which  is  attached,  by 
means  of  twine,  or  a  strong  rubber  ring,  a  wide-mouthed  bottle. 
Any  quantity  of  water  may  be  poured  into  the  bag,  and  all  the 
objects  which  it  contains  will  roll  down  the  sides  of  the  bag 
and  fall  into  the  bottle,  while  the  fluid  escapes  through  the 
sides.  Delicate  objects  are  consequently  not  exposed  to 
pressure,  rubbing,  or  any  other  violence,  as  they  would  be  in 
an  ordinary  filter  or  bag,  and  the  whole  affair  is  so  simple  that 
any  one  can  make  it. 

A  slight  modification  of  this  arrangement  will  be  found  ad- 
mirably adapted  to  the  microscopic  examination  of  the  water 
supplied  to  cities.  The  bag  may  be  attached  to  any  faucet,  and 
and  all  the  water  that  is  used  in  the  household  may  be  caused 
to  pass  through  it.  In  this  case,  if  the  bag  be  made  of  some 
tolerably  stout  material,  it  may  be  firmly  tied  to  the  faucet, 
and  then  all  the  water  that  is  consumed  will  be  very  thor- 
oughly purified. 

Another  very  excellent  device  is  the  bottle  invented  by 
Mr.  Wright,  of  which  a  modified  form  is  shown  in  Fig. 
59.  The  mouth  of  the  bottle  is  closed  by  means  of  a 
co,k  in  which  two  funnels  are  inserted. 
One  of  these  funnels  is  placed  in  the  bot- 
tle, mouth  down;  the  other  projects  above 
the  cork,  as  shown  in  the  engraving.  The 
mouth  of  the  funnel  that  is  in  the  bottle 
is  covered  with  muslin  or  flannel,  held  in 
place  by  a  rubber  band,  which  is  prevented 
by  a  wire  ring  from  slipping  along  the  con- 
ical surface  of  the  funnel.  When  water 
is  poured  into  the  other  funnel,  it  passes 
into  the  bottle  until  the  latter  is  full,  and 
then  it  flows  out  of  the  first  funnel,  and 
is  carried  off  by  means  of  a  short  piece  of 
rubber  tubing.  Meanwhile,  all  solid  parti- 
cles are  held  back  by  the  filter,  and  as  the 
latter  is  horizontal  and  with  the  filtering  Fig.  59. 


OF    THE    MICEOSCOPE.  185 

surface  downwards,  most  objects  of  interest  fall  away  from  it, 
and  may  be  found  in  the  water.  A  single  bottle  of  this  descrip- 
tion, is  sufficient,  as  the  cork  is  easily  removed,  so  that  the  water 
may  be  poured  into  other  bottles.  As  ordinarily  made  and  sold, 
Wright's  collecting  bottle  is  an  expensive  piece  of  apparatus, 
costing  four  or  five  dollars,  but  as  shown  in  the  engraving  it 
may  be  made  for  a  few  cents  by  any  tinsmith. 

Where  it  is  desirable  to  keep  the  specimens  thus  obtained  so 
that  they  may  be  examined,  and  their  life-history  studied, 
bottles  and  jars  of  almost  any  kind  may  be  used,  but  those  which 
we  have  found  most  convenient  are  what  are  known  as  "quin- 
ine "  bottles,  and  may  be  had  at  most  druggists.  For  ordinary 
objects  they  are  just  about  the  right  size,  and  as  they  are  made 
of  tolerably  clear  glass  it  is  easy  to  examine  the  objects  through 
the  sides  of  the  bottle.  A  dozen  or  two  of  these  little  aquaria 
occupy  very  little  space,  arid  are  easily  handled.  Great  care 
must  in  general  be  taken  to  exclude  from  the  vessels  contain- 
ing the  finer  organisms,  such  predatory  animalcules  as  devour 
them.  Water  fleas,  the  larvae  of  insects,  etc.,  will  soon  make 
away  with  the  finer  specimens.  On  this  account  great  diffi- 
culty is  found  in  keeping  the  Vohox  Globator,  since  it  is  greedily 
devoured  by  various  rotifers,  and  these  are  exceedingly  difficult 
to  exclude.  We  have  succeeded  best  in  this  case  by  partially 
filling  a  bottle  with  well-filtered  water  taken  from  the  same 
pool  as  the  specimens,  and  transferring  the  objects  to  it  singly 
so  as  to  avoid  transferring  their  enemies  too.  For  this  purpose 
the  dipping  tube  should  be  used.  Some  authors  caution  us 
against  mixing  the  inhabitants  of  different  pools,  on  the  ground 
that  being  strangers  to  each  other  they  will  fight.  This  is  more 
fanciful  than  accurate,  though  it  has  a  basis  of  truth.  It  is  not 
the  circumstance  that  they  are  strangers  that  causes  the  diffi- 
culty, but  the  fact  that  the  one  is  the  natural  prey  of  the  other. 
The  same  thing  occurs  between  inhabitants  of  the  same  pool. 
It  must  be  remembered,  however,  that  very  slight  changes  in 
the  conditions  in  which  they  are  placed  will  often  cause  the  de- 
struction of  these  objects.  Thus,  we  have  seen  some  very  fine 
gatherings  totally  destroyed  by  being  removed  from  soft,  boggy 
water  to  clear,  hard  well  water.  Therefore,  in  transferring 
either  animals  or  vegetables  to  an  aquariun,  it  is  well  to  supply 


186  SELECTION   AND    USE 

them  with  the  same  water,  mud,  etc.,  in  which  they  were  origi- 
nally found.  It  will  sometimes,  however,  be- well  to  filter  the 
water  so  as  to  remove  all  such  inhabitants  as  are  apt  to  eat  up 
the  others.  Water,  may  be  filtered  through  paper,  or  where  fil- 
tering paper  is  inaccessible,  the  neck  of  a  funnel  may  be  loosely 
plugged  with  cotton.  Even  this  does  not  quite  free  it  from 
noxious  eggs  or  germs,  and  we  have  sometimes  boiled  it  in  the 
flask  in  which  the  objects  were  to  be  kept.  The  mouth  of  the 
flask  was  then  plugged  with  loose  cotton,  and  when  the  water 
was  cold,  the  objects  we  wished  to  preserve  were  introduced. 

When  floating  freely  in  these  diminutive  aquaria,  many  ob- 
jects are  as  difficult  to  find  and  capture  as  would  be  a  small  fish 
in  a  large  pond.  The  microscopist,  therefore,  requires  special 
means  for  capturing  them,  and  placing  them  on  a  slide.  For 
this  purpose  nothing  serves  so  well  as  what  are  known  as  dip- 
ping or  fishing  tubes.  These  are  simply  glass  tubes  of  different 
diameters  (from  one  tenth  to  one  quarter  of  an  inch),  and  of 
any  convenient  length.  They  are  used  by  closing  the  upper 
end  with  the  finger,  bringing  the  lower  end  near  the  object  (un- 
der the  water) ,  and  then  removing  the  fingers  from  the  uppei 
end.  The  water,  in  seeking  to  find  its  own  level  in  the  tube 
rushes  in  with  great  force  and  carries  the  object  with  it.  By 
again  placing  the  finger  on  the  upper  end  of  the  tube,  the  latter 
may  be  lifted  from  the  bottle,  and  the  water  with  it,  and  by  a  little 
dexterous  management  it  is  easy  to  cause  the  object  to  flow  out 
on  a  slide  without  allowing  too  much  water  to  go  with  it. 
These  tubes  are  made  straight,  curved,  and  with  one  end  drawn 
to  a  point,  but  for  most  purposes  the  straight  tube  answers  best 
as  it  is  most  easily  kept  clean.  We  prefer  to  grind  the  ends 
rather  than  to  make  them  smooth  by  fusion,  as  the  latter  pro- 
cess generally  contracts  the  opening,  and  renders  the  tube  dif- 
ficult to  clean.  The  best  plan,  however,  is  to  heat  the  upper 
end  strongly  before  the  blowpipe,  and  turn  the  edge  outward 
like  the  mouth  of  a  test-tube.  It  is  then  easily  closed,  and  the 
tube  is  very  strong.  The  lower  end  should  be  ground. 


01'    THE  MICEOSCOPE.  187 

PREPARATION,  PRESERVATION  AND  MOUNTING  OF 
OBJECTS. 

These  three  operations  are  so  frequently  applied  as  a  single 
process  to  objects,  that  many  writers  have  failed  to  make  a  suf- 
ficient distinction  between  them.  By  keeping  the  proper  dis- 
tinction clearly  in  mind,  however,  the  student  will  not  only 
save  much  valuable  time,  but  he  will  secure  vastly  better  re- 
sults. Except  by  those  who  are  more  anxious  to  increase  the 
number  of  objects  in  their  cabinet  than  the  amount  of  know- 
ledge which  they  possess,  a  very  large  proportion  of  the  ob- 
jects examined  will  never  be  preserved  or  mounted  at  all.  This 
however,  should  not  prevent  the  utmost  care  being  given  to  the 
process  of  preparing  them  for  thorough  examination.  On  the 
other  hand  it  often  happens  that  objects  which  have  been 
carefully  prepared  and  mounted,  spoil  because  they  have  not 
been  subjected  to  a  proper  preserving  process.  Hence  the 
importance  of  treating  these  operations  separately  and  fully. 

The  Preparation  and  Examination  of  Objects.— 

It  is  a  common  but  very  erroneous  idea  that  the  only  thing 
that  is  necessary  in  order  to  examine  any  object  under  the  mi- 
croscope, is  simply  to  place  it  on  the  stage,  and  get  it  into 
focus.  With  the  exception  of  mounted  objects,  a  very  few 
transparent  objects — such  as  the  wings  of  insects — and  some 
things  that  are  viewed  by  reflected  light,  every  substance  re- 
quires to  undergo  careful  preparation  before  it  can  be  fit  for 
profitable  examination.  A  good  example  of  the  necessity  for 
such  preparation  is  seen  in  the  common  potato,  a  piece  of  which 
when  simply  placed  on  the  stage  of  the  microscope,  and  brought 
into  focus,  appears  as  a  glistening  mass,  and  reveals  nothing  of 
its  true  structure.  If  we  now  cut  from  this  lump,  by  means  of 
a  very  sharp  knife,  an  exceedingly  thin  slice,  place  it  on  a 
plate  of  glass,  moisten  it  with  a  little  spirit  and  water,  or  better 
still,  glycerine  and  water,  and  place  over  it  a  thin  glass  cover,  it 
will  disclose  to  us  a  most  wonderful  and  beautiful  structure.  The 
entire  mass  will  be  seen  to  be  composed  of  cells,  these  cells  be- 
ing filled  with  granules  of  starch  of  various  sizes. 


188  SELECTION   AND    USE 

The  operation  which  we  have  thus  briefly  described  as  appli- 
cable to  the  potato,  is  required  for  a  great  many  other  materials ; 
for  whenever  a  substance  is  to  be  examined  under  any  except 
the  very  lowest  powers,  it  is  absolutely  necessary  to  obtain  it  in 
pieces  as  thin  as  possible,  so  that  the  light  may  readily  pass 
through  them,  and  it  is  in  general  requisite  to  increase  their 
transparency  either  by  immersing  them  in  a  fluid,  or  by  some 
other  means.  In  preparing  objects  for  the  microscope,  our 
aim  is  in  general  to  examine  either  the  ultimate  structure  or 
the  substance  under  investigation,  or  the  arrangement  of  its 
different  parts;  and  the  processes  which  are  most  available  for 
this  purpose  may  be  classed  under  three  heads:  1,  Mechanical, 
such  as  section-cutting,  dissection  and  injection;  2,  Chemical, 
such  as  the  use  of  iodine  for  detecting  starch ;  of  alcohol  for 
hardening  certain  structures;  of  coloring  substances  for  stain- 
ing germinal  matter,  etc.;  3,  Optical,  such  as  the  action 
whereby  certain  liquids  change  the  transparency  of  some  ob- 
jects* Of  some  of  these  processes,  such  as  injection,  staining 
and  the  extended  use  of  chemical  tests,  elaborate  descriptions 
would  be  required  in  order  to  enable  the  student  to  carry  them 
out  with  success,  and  we  must  refer  him  to  the  works  of  Beale 
and  Frey,  which  are  very  complete  on  these  points. 

Thin  sections  of  any  soft  substance  are  easily  made  with  a 
very  sharp  knife — a  good  razor  being  probably  the  best  availa- 
ble instrument.  For  work  in  the  higher  departments  of  micro- 
scopy, and  for  the  preparation  of  fine"  objects  for  sale,  special 
instruments  known  as  section-cutters  are  employed,  but  for  the 
ordinary  work  of  investigation,  they  are  not  absolutely  neces- 
sary though  very  convenient.  Using  a  good  sharp  razor,  it  is 
an  easy  matter  to  shave  off  any  soft  substance  a  wedge  shaped 
piece,  the  edge  of  which  thins  off  to  nothing,  and  which  pre- 
1  sents  in  its  different  parts  all  varieties  of  thickness,  so  as  to  afford 
a  perfect  opportunity  to  study  the  object  under  examination.  In 
this  way,  which  is  known  as  the  "  free-hand  "  method,  suitable 
sections  of  most  animal  and  vegetable  substances  may  easily  be 
prepared,  and  the  student  will  be  surprised  at  the  dexterity 
which  a  little  care  and  practice  will  confer. 

For  cutting  sections  of  very  soft  tissues  a  special  knife,  known 
as  Valentin's  knife,  has  been  invented.  It  consists  of  two 


OF   THE  MICROSCOPE.  189 

blades  so  arranged  in  one  handle  that  their  distance  from  euch 
other  may  be  easily  regulated.  When  a  cut  is  made  with  this 
double-bladed  knife,  a  thin  slice  of  the  tissue  passes  between 
the  blades,  and  constitutes  the  section.  It  is  an  instrument, 
however,  which  will  hardly  be  used  by  beginners.  Sections 
of  substances  of  greater  consistence,  such  as  wood  and  soft 
bones,  are  most  easily  made  in  a  regular  section  cutter.  The 
patterns  according  to  which  these  instruments  are  constructed 
are  very  various,  but  they  all  act  on  the  principle  of  raising 
above  the  surface  of  a  brass  table,  by  means  of  a  fine  screw,  the 
substance  to  be  cut,  and  then  passing  a  very  sharp  razor  or 
knife  over  the  table  so  as  to  shave  off  the  projecting  part  of  the 
object.  The  table  is  usually  of  brass,  ground  and  polished. 
This  gives  rise  to  two  serious  defects.  The  metal  is  too  soft  in 
the  first  place,  so  that  it  is  impossible  to  press  with  sufficient 
force  on  the  razor  without  cutting  into  the  table,  and  secondly, 
when  any  soft  metal  has  been  .ground  on  a  grindstone  or  emery 
wheel,  the  surface  becomes  so  impregnated  with  gritty  matter, 
that  it  very  rapidly  destroys  the  edge  of  the  cutting  tool.  We 
avoid  these  difficulties  by  fitting  to  our  section  cutter  a  stout 
plate  of  hardened  steel,  the  surface  of  which  has  been  highly 
polished  by  means  of  buff  leather.  Quekett  describes  a  cut- 
ting machine  in  which  the  difficulties  we  have  mentioned  are 
obviated  by  fixing  the  knife  in  a  frame  so  that  it  is  raised  above 
the  table,  and  does  not  touch  the  metal.  Its  edge  is  thus  pre- 
served from  injury,  and  the  blade  itself  cannot  be  affected  by 
variations  in  the  pressure  exerted.  Dr.  Curtis,  of  this  city,  has 
adopted  the  same  principle  in  his  section  cutter,  the  details  of 
which  are  admirably  carried  out. 

In  making  sections  of  wood  and  similar  substances,  the  speci- 
men is  first  well  soaked  in  dilute  alcohol,  and  is  then  fastened 
securely  into  the  tube  of  the  section  cutter,  either  by  wedges  or 
by  casting  wax  or  paraffin  around  it.  The  process  of  raising  it 
by  means  of  the  screw  and  passing  the  knife  over  it,  is  simple 
enough,  and  can  easily  be  learned. 

With  the  ordinary  cutting  machine,  success  in  making  thin 
sections  seems  to  depend  upon  the  perfect  sharpness  of  the 
cutting  edge,  the  thorough  moistening  of  the  knife  and  section, 
ind  the  rigidity  of  the  blade.  The  latter  point  frequently  fails 


190  SELECTION    AND    tJSH 

to  receive  the  attention  that  it  deserves.  Where  a  thin,  flexi- 
ble blade  is  used,  a  moderate  change  in  the  amount  of  pressure 
employed  will  make  a  great  difference  in  the  thickness  of  the 
section,  even  so  far  as  to  double  it.  When  the  blade  is  stiff,  a 
change  in  the  degree  of  pressure  has  but  little  effect. 

Soft  substances  must  first  be  hardened  either  by  immersion 
in  alcohol  or  other  means,  and  in  general  must  be  supported  by 
being  surrounded  with  melted  wax  or  paraffin.  Where  the 
specimen  is  very  slender  (such  as  a  hair)  it  must  be  carefully 
supported  between  firm  and  rigid  clamps.  Corks  and  similar 
yielding  substances,  which  are  recommended  in  most  books, 
never  give  a  cross  section  accurately  taken  at  right  angles.  The 
same  is  true  of  the  plan  so  much  recommended  for  obtaining 
sections  of  hair,  viz. :  to  pass  the  razor  over  the  face  shortly 
after  shaving.  We  get  sections  it  is  true,  but  they  are  all 
oblique.  The  best  way  to  get  true  sections  is  to  imbed  the  sub- 
stances in  glue,  gum,  paraffin,  wax  or  some  such  material. 

Sections  of  bone  are  prepared  by  sawing  off  a  thin  slice  in  the 
first  place,  and  cementing  it  to  a  slide  by  means  of  thick  or  old 
balsam;  one  side  is  then  filed  or  ground  flat,  and  polished  on 
buff  leather,  after  which  the  section  is  transferred  to  another 
slide  so  as  to  expose  the  other  side,  which  is  then  filed  down 
and  polished  as  before.  Great  care  must  be  taken  so  as  to  hit 
just  the  right  thickness,  and  the  operation  of  cementing  to  the 
slide  must  be  performed  expeditiously,  so  that  the  balsam  may 
not  saturate  the  section,  and  render  it  too  transparent,  as  when 
this  occurs  certain  very  important  features  become  invisible. 

Very  hard  substances  require  special  apparatus,  and  consid- 
erable skill.  Still  it  is  astonishing  what  may  be  accomplished 
by  means  of  good  files,  whetstones  and  grindstones  in  the  way 
of  preparing  thin  and  transparent  sections  even  of  such  sub- 
stances as  rocks  and  stones. 

In  order  to  acquire  correct  ideas  in  regard  to  the  structure  of 
objects,  of  which  sections  are  examined,  the  student  should  fa- 
miliarize himself  with  the  geometrical  forms  produced  by  cat- 
ting cylinders,  cones,  spheroids,  etc.,  in  various  directions. 
Thus  a  cylindrical  vessel,  cut  square  across,  shows  a  circle;  when 
cut  obliquely  it  shows  an  oval  (ellipse)  of  greater  or  less  length 
and  when  cut  longitudinally  it  shows  two  lines  which  have  no 


OF  THE  MICKOSCOPE.  191 

u,ppd,rent  connection  with  each  other.  The  truth  is,  however, 
that  we  should  never  deduce  the  form  of  vessels  from  sections 
alone.  In  every  case  it  is  necessarr  to  examine  carefully  dis- 
sected preparations  as  well  as  sections 

The  soft  parts  of  animals  and  vegetables  are  frequently  pre- 
pared for  examination  by  careful  dissection,  that  is  to  say  the 
different  parts  are  separated  from  each  other,  and  freed  from  ex- 
traneous matter  by  means  of  knives,  scissors,  forceps,  needles, 
camel  hair  pencils,  etc.  The  knives  used  by  the  microscopist  are 
similar  to  the  scalpels  ordinarily  employed  by  anatomists,  but 
smaller,  and  unless  very  finely  tempered  and  well-sharpened, 
they  are  worthless.  The  knives  sent  out  with  low  priced  micro- 
scopes are  in  general  the  veriest  trash,  and  the  same  is  true  of  the 
needles.  There  are  three  kinds  of  scissors  which  the  microscop- 
ist will  find  useful — plain,  straight  scissors,  elbow  scissors,  and 
carved  scissors.  They  must  be  small,  sharp  and  well  made.  But 
the  most  useful,  as  well  as  the  simplest  instruments  for  dissecting 
are  a  pair  of  needles,  or,  rather,  a  needle  and  a  very  fine 
spatula.  The  needles  used  are  those  ordinarily  employed  by 
seamstresses  •,  they  should  be  fixed  in  a  light  wooden  handle  and 
carefully  polished.  The  latter  is  a  most  important  point,  for 
it  will  be  found  that  ordinary  needles  are  too  rough  for  deli- 
cate work,  as  may  be  easily  seen  by  examining  them  under  the 
microscope.  For  microscopical  purposes  needles  are  made 
both  straight  and  curved — the  latter  being  a  very  useful  form. 
In  order  to  bend  a  needle,  it  must  first  be  heated  in  the  flame 
of  a  candle,  then  bent  by  proper  pliers,  after  which  it  must  be 
carefully  re-tempered.  There  is  little  danger  of  getting  it  too 
hard,  provided  it  is  not  burned.  After  being  hardened  it  must 
be  carefully  re-polished.  The  handles  should  be  light  and 
smooth.  Ordinary  penholders  make  good  handles  and  cost  but 
a  trifle,  but  in  case  of  need  any  piece  of  straight-grained,  light 
wood  will  answer.  Universal  handles,  handles  with  ferrules, 
handles  wound  with  thread,  etc,,  look  as  if  they  were  not  com- 
mon articles,  and  are  purchased  by  many,  but  no  working  mi- 
croscopist would  give  them  table-room.  All  the  so-called  uni- 
versal handles  in  market  are  too  -clumsy  and  heavy. 

In  using  needles  or  knives  for  dissection,  they  are  generally 
used  in  pairs,  that  in  the  right  hand  being  used  for  teasing  or 


192  SELECTION    AND    tJSE 

cutting,  while  the  one  in  the  left  hand  is  used  for  holding  the 
object  firmly  in  its  place.  For  the  latter  purpose,  however,  we 
prefer  a  very  narrow  spatula,  curved  and  highly  polished. 
Curved  needles,  with  the  curve  placed  flat,  answer  very  well, 
however. 

For  the  removal  of  loose  matter,  and  for  arranging  parts 
which  have  been  dissected  out,  there  is  nothing  more  useful 
than  good  camel  hair  pencils.  Indeed,  they  are  indispensable, 
and  with  needles  and  pencils — two  of  the  simplest  and  cheapest 
articles — it  is  possible  to  do  almost  everything. 

During  the  process  of  dissection  the  object  must  be  supported 
upon  a  glass  plate  or  a  dissecting  pan,  according  to  its  size.  Some 
of  the  finest  preparations  have  been  worked  up  on  ordinary  slides 
three  inches  long  by  one  wide,  and  as  it  is  almost  always  neces- 
sary to  have  the  object  covered  with  liquid,  a  single  drop  suf- 
fices in  this  case.  But  where  larger  objects  are  to  be  dissected, 
ordinary  slides  are  not  large  enough,  and  besides  there  is  no 
provision  made  for  holding  a  sufiicient  quantity  of  liquid. 
Various  kinds  of  dissecting  dishes  or  pans  have  therefore  been 
devised.  Those  used  by  the  author  are  exceedingly  simple 
and  cheap,  and  are  shown  in  Fig.  60.  We  use  three  kinds, 
two  with  opaque  bottoms,  and  one 
in  which  the  bottom  is  transparent. 
The  latter  is  used  for  objects  which 
are  transparent,  and  is  precisely  like 
the  others,  except  that  a  portion 
Fig.  60.  of  the  metal  bottom  is  cut  away  and 

a  piece  of  plate  glass  cemented  over 

the  aperture.  Those  used  for  opaque  objects  are  simply 
oblong  tin  dishes,  each  two  inches  long,  one  and  a  quarter 
wide  and  half  an  inch  deep.  The  bottom  plate  extends  on  each 
side,  so  as  to  form  rests  for  the  fingers,  by  which  the  pan  may 
be  kept  steady.  Into  this  pan  is  poured  a  mixture  of  equal 
parts  of  resin  and  beeswax,  softened  if  necessary  with  a  little 
lard.  It  should  be  just  so  soft  that  a 'pin  may  be  easily  stuck 
into  it,  and  this  affords  us  the  means  of  pinning  out  the  different 
parts  of  a  dissection  as  we  progress.  In  one  dish  the  wax  is 
colored  black  with  lampblack,  and  this  forms  a  wonderfully 
f  ffective  back  ground  for  most  objects;  the  wax  in  the  other 


Off  THE  MICROSCOPE.  193 

pan  is  white,  chalk  or  sulphate  of  baryta  being  substituted  for 
lampblack.  The  pan  with  a  transparent  bottom  is  of  precisely 
the  same  size,  except  that  the  depth  is  but  halt'  as  much — the 
extra  depth  in  the  other  pan  being  filled  with  wax.  A  quarter 
of  an  inch  is  a  sufficient  depth  of  liquid  for  most  objects,  and 
when  the  sides  of  the  pan  are  higher  than  necessary  they  inter- 
fere with  the  use  of  knives  and  needles.  y 

Dissections  may  also  be  carried  on  in  watch-glasses,  though  they 
are  not  quite  as  convenient  as  pans  with  perfectly  flat  bottoms. 
The  kind  known  as  lunette  glasses  should  be  chosen,  as  they 
are  flat  in  the  centre.  When  a  watch-glass  is  used  for  this  pur- 
pose, it  is  necessary  to  cement  it  into  a  hole  cut  in  a  thin  piece 
of  wood  about  four  inches  long,  and  of  a  width  which  is  rather 
greater  than  the  diameter  of  the  glass. 

Most  of  this  work  is,  of  course,  done  under  a  simple  micro- 
scope. The  Excelsior,  when  screwed  to  a  larger  base,  as  de- 
scribed on  page  42,  answers  very  well.  Larger  and  more  ex- 
pensive dissecting  microscopes  are  supplied  by  most  opticians. 

In  addition  to  these  general  methods,  which  are  applicable 
to  a  great  variety  of  subjects,  there  are  a  few  special  processes 
which  must  be  adopted  in  particular  cases.  In  some  instances, 
as  when  the  line  of  investigation  is  a  new  one,  the  microscop- 
ist  must  work  out  his  own  processes,  but  the  following  special 
cases  will  probably  prove  interesting  to  beginners. 

It  frequently  happens  that  the  objects  for  which  the  micro- 
scopist  is  searching  are  found  mixed  with  coarser  materials, 
and  in  this  case  it  will  be  found  possible  to  effect  a  separation 
by  the  process  known  as  elutriation  or  washing.  Mix  the  mat- 
ter thoroughly  with  water  in  a  tall  jar  and  allow  it  to  sett'e. 
In  a  short  time — say  one  minute — the  very  coarse  particles  will 
have  fallen  to  the  bottom,  and  if  the  liquid  be  now  poured  off 
and  allowed  to  settle,  the  finer  portion  will  be  found  in  the 
second  vessel.  By  graduating  the  time  and  carrying  the  pro- 
cess out  to  its  full  extent,  a  wonderfully  perfect  separation  may 
be  effected.  Diatomaceous  earth  may  frequently  be  treated  in 
this  way  to  advantage. 

In  some  cases  separation  must  be  effected  by  burning,  or  the 
action  of  chemical  agents.  Guano  and  various  organic  matters 
yield  interesting  residues  alter  everything  soluble  has  been 


194  SELECTION   AND    USE 

washed  away  and  everything  combustible  has  been  burnt  either 
with  fire  or  nitric  acid.  So  too  the  siliceous  cuticles  of  plants 
may  be  procured  by  destroying  all  the  other  parts  by  chemical 
means.  The  best  way  is  to  heat  them  in  nitric  acid,  and  add 
to  the  hot  liquid  a  small  quantity  of  powdered  chlorate  of  pot- 
ash. The  quantities  used  must  be  very  small,  and  great  care 
must  be  exercised. 

It  is  frequently  necessary  to  separate  a  small  quantity  of1 
deposit  from  a  large  amount  of  liquid,  filtering  being  inadmis- 
sible. For  this  purpose  use  a  conical  glass  or  a  large  test  tube, 
allow  plenty  of  time  for  the  deposit  to  settle,  and  give  occa- 
sionally a  slight  stir,  so  as  to  detach  the  particles  from  the 
sides  of  the  vessel.  Then  pass  a  large  dipping  tube  (one  quar- 
ter of  an  inch  in  diameter)  to  the  bottom,  the  upper  end  of  the 
tube  being  closed  with  the  finger.  On  withdrawing  the  finger 
the  liquid  and  deposit  rush  in.  Have  ready  a  small  ball  of 
soft  cement  (resin  and  beeswax  equal  parts,  softened  with  oil) 
and  with  it  close  the  upper  end  of  the  tube,  which  may  now  be 
withdrawn,  carrying  the  liquid  with  it.  Place  the  tube  in  a 
vertical  position,  with  its  lower  end  on  a  slide  or  in  a  watch- 
glass,  and  support  it  either  by  means  of  the  ring  of  a  small 
retort  stand  or  by  a  simple  wire-  having  a  ring  (horizontal)  at 
the  upper  end,  and  a  small  piece  of  board  for  a  foot.  Beale 
directs  us  to  cork  the  tube,  but  this  is  difficult  unless  the  tube 
is  made  specially  for  the  purpose  with  a  mouth  like  that  of  a 
test  tube.  Tubes  made  in  this  way  are,  however,  the  most  con- 
venient, and  a  good  velvet  cork  closes  them  perfectly. 

There  is  a  class  of  insect  preparations,  which  are  quite  inter- 
esting, though  they  are  not  as  instructive  as  inferior  prepara- 
tions made  by  the  process  of  dissection.  We  refer  to  the  whole 
insects  found  in  most  collections.  They  are  prepared  by  soak- 
ing the  insect  in  liquor  potassse,  which  may  be  had  from  any 
druggist;  this  renders  the  internal  organs  soluble  and  the  outer 
horny  skeleton  transparent.  The  viscera  are  then  expelled  by 
pressure  with  a  camel  hair  pencil,  the  insect  well  washed  in  pure 
water,  soaked  first  in  alcohol,  and  then  in  turpentine,  and 
finally  mounted  in  balsam.  The  points  requiring  attention  are 
these:  Soaking  just  the  right  length  of  time  in  the  potash,  for  if 
the  insect  remains  too  long  in  this  liquid  it  will  be  destroyed ; 


OP    THE   MICKOSCOPE.  195 

allowing  plenty  of  time  for  the  alcohol  to  displace  the  water, 
and  for  the  turpentine  to  displace  the  alcohol;  and  manipula- 
ting the  insect  with  great  care,  so  as  not  to  break  any  of  the  parts. 
The  eyes  of  insects  are  prepared  by  macerating  them  in  very  weak 
potash,  and,  while  still  soft,  pressing  them  between  two  slips  of 
glass.  If  allowed  to  harden  before  being  pressed  they  will 
split  at  the  edges.  The  handsomest  preparations  of  eyes  are 
obtained  by  taking  a  thin  slice  from  a  large  eye,  such  as  that 
of  a 'dragon  fly,  and  treating  it  as  directed. 

The  feet  of  insects  are  in  general  easily  prepared.  Moderate 
soaking  in  potash,  careful  washing  in  water,  thorough  soaking 
in  alcohol  and  turpentine,  and  careful  management  in  properly 
displaying  them  on  the  slide,  are  the  secrets  of  success.  The 
student  who  wishes  to  make  a  careful  study  of  these  objects, 
however,  should  place  them  in  glycerine,  after  soaking  them  in 
potash  and  thoroughly  washing  them.  They  should  of  course 
be  deposited  in  a  cell  filled  with  liquid,  and  then  covered  with 
thin  glass,  and  examined.  The  so-called  tongues,  etc.,  of  in- 
sects require  no  potash,  being  sufficiently  transparent  without 
it,  and  after  being  soaked  successively  in  alcohol  and  turpen- 
tine, they  may  be  mounted  in  balsam.  When  wanted  for  exam- 
ination merely,  immerse  them  in  dilute  glycerine,  and  if  the 
student  can  succeed  in  mounting  them  in  cells,  in  glycerine  or 
some  of  the  gelatinous  media  hereafter  described,  they  will  show 
their  structure  to  far  better  advantage  than  in  balsam. 

In  determining  the  character  of  what  is  brought  into  view  by 
the  processes  detailed,  great  aid  will  be  derived  from  the  use 
of  chemical  tests.  Thus,  in  the  case  of  the  potato,  previously 
described,  most  persons  who  had  read  anything  at  all  upon  such 
sub  j  ects,  would  recognize  the  starch  granules.  All  starch  granules, 
however,  are  not  of  the  same  form  as  those  found  in  the  potato; 
indeed,  some  would  hardly  be  recognized  at  all,  except  by  those 
having  considerable  experience.  But  if  a  little  of  the  tincture 
of  iodine  be  brought  into  contact  with  them,  they  at  once  be- 
come deeply  blue.  This  subject  is  too  extensive  to  be  dis- 
cussed here,  but  those  who  desire  to  become  proficient  in  the 
use  of  the  microscope  cannot  safely  neglect  it. 

In  most  cases  after  an  object  has  been  carefully  brought  into 
proper  mechanical  condition,  in  one  of  the  ways  we  have  de- 


196  SELECTION  AND    USE 

sc.-ibod,  it  is  necessary  to  immerse  it  in  some  suitable  medium, 
so  as  to  render  it  clear  and  transparent.  The  action  of  such 
media  may  be  very  well  illustrated  by  the  following  experiment : 
Take  a  short  piece  of  black  human  hair,  place  it  on  a  slide, 
bring  it  into  focus  and  examine  it.  It  will  appear  as  a  dark 
cord  with  a  light  line  running  down  the  centre,  and  from  this 
circumstance  has  arisen  the  erroneous  popular  idea  in  regard  to 
the  tubular  structure  of  hair.  Apply  a  drop  of  glycerine  di- 
luted with  an  equal  bulk  of  water,  and  again  examine  it.  The 
appearance  will  have  entirely  changed,  having  become  clearer 
and  more  definite,  so  that  the  structure  of  the  hair  is  more 
easily  made  out.  This  effect  depends  upon  the  refracting  power 
of  the  liquid  used.  The  following  liquids  are  usually  employed 
for  this  purpose,  their  efficiency  being  in  direct  ratio  to  their 
index  of  refraction,  which  we  append  to  each.  Water,  1.336; 
glacial  acetic  acid,  1.38;  alcohol,  1.372;  vitreous  humour,  1,340; 
sea-water,  1.343;  equal  parts  of  glycerine  and  water,  1.40;  pure 
glycerine,  1.475;  oil  of  turpentine,  1.478;  Canada  balsam, 
1.532—1.549;  bisulphide  of  carbon,  1.678;  oil  of  annis,  1.811. 
Alcohol  and  water,  and  solutions  of  various  salts  in  water  are 
also  very  useful.  When  a  pure  article  of  glycerine  is  not  avail- 
able, a  solution  of  white  sugar  may  be  used  with  good  results. 

Great  care  must  be  exercised  lest  the  fluid  that  is  added 
should  change  the  form  or  structure  of  the  object.  Upon  this 
subject  the  remarks  of  Frey  are  very  judicious.  He  says :  ' '  Theory 
requires  that  each  constituent  of  the  body  should  be  examined 
in  a  fluid  medium  which  resembles  in  respect  to  quality  and 
quantity,  the  fluid  which  saturates  the  living  tissue.  Naturally 
this  requirement  cannot  be  completely  fulfilled  in  practice;  our 
aim  should  be  to  approach  it  as  nearly  as  possible.  Saliva, 
vitreous  humour,  amniotic  liquor,  serum  and  dilated  albumen 
are  generally  recommended  as  suitable  media  for  the  investiga- 
tion of  delicate  changeable  tissues,  and,  in  certain  cases,  they 
accomplish  their  object  in  a  satisfactory  manner.  But  do  not 
expect  them  to  suffice  for  every  case.  Not  unfrequently  one 
and  the  same  tissue  of  different  species  of  animals  reacts  differ- 
ently with  the  same  fluid  medium,  as  may  be  seen  with  the 
blood  corpuscles.  M.  Schultze  has  communicated  to  us  an  im- 
portant and  readily  proved  observation  of  Landolt's,  that  ani- 


OF   THE   MICROSCOPE.  197 

mal  fluids  may  be  preserved  from  decomposition  for  a  long  time 
by  the  addition  of  a  small  piece  of  camphor." 

Schultze  recommends  as  a  neutral  fluid,  suitable  for  most 
tissues,  a  liquid  which  he  calls  "  lod-serum.*'  It  consists  of  the 
amniotic  fluid  of  the  calf,  to  which  has  been  added  a  concen- 
trated tincture  of  iodine  or  a  strong  solution  of  iodine  in  the 
proportion  of  six  drops  to  the  ounce.  The  color  of  the  solution 
is  at  first  wine  yellow,  but  after  a  few  hours  it  becomes  paler « 
this  paleness  afterwards  increases,  and  the  subsequent  addition 
of  a  few  drops  of  the  iodine  solution  becomes  necessary.  As 
the  amniotic  fluid  is  not  always  attainable,  a  good  substitute 
may  be  prepared  by  mixing  1  ounce  white  of  egg,  9  ounces 
water,  and  40  grains  chloride  of  sodium,  with  the  proper  pro- 
portion  of  tincture  of  iodine. 

During  the  entire  process  of  preparation,  the  greatest  atten- 
tion must  be  paid  to  cleanliness.  Particles  of  dust,  which  to  the 
unassisted  vision  are  invisible,  become  offensively  prominent 
under  the  microscope.  To  exclude  these,  and  to  protect  the 
objects,  it  is  important  that  the  latter  should  be  kept  carefully 
covered  when  not  actually  undergoing  some  operation.  Small 
bell  glasses  are  recommended  for  this  purpose  by  Dr.  Carpenter, 
and  they  answer  admirably.  We  prefer,  however,  as  being 
cheaper  and  less  bulky,  watch  glasses  to  which  a  handle  has 
been  cemented  as  shown  in  Fig.  61.  The 
handle  may  be  a  little  knob,  turned  out  of 
a  piece  of  wood,  or  where  this  is  not  conveni- 
ent a  small  cork  will  answer.  A  little  sealing 
Fig.  61  wax  serves  for  a  cement,  the  watch  glass  be- 

ingf  heated  before  the  wax  is  applied.  Flat 
plates  of  glass  answer  well  to  cover  the  dissecting  pans  previ- 
ously described. 

When  a  number  of  objects  are  to  be  protected  for  some  time, 
we  place  them  on  a  piece  of  plate  glass  eight  inches  square, 
cover  each  with  a  watch-glass  cover,  and  protect  the  whole  by 
means  of  a  bell  jar  with  ground  edges.  The  latter  fits  closely 
to  the  plate  glass  and  excludes  everything,  while  the  small  covers 
protect  the  individual  specimens  when  the  large  cover  is  raised 
for  the  purpose  of  getting  at  them. 
Singular  mistakes  have  arisen  from  the  fact  that  foreign 


198  SELECTION    AND    USE 

bodies  which  have  accidentally  found  their  way  into  a  prepara- 
tion have  been  mistaken  for  part  of  the  specimen.  The  only 
way  to  avoid  similar  errors  is  to  exclude  all  such  intruders  by 
means  of  proper  covers,  and  to  become  familiar  with  them  so 
that  they  may  be  instantly  recognized  when  present.  Dr.  Beale 
gives  the  following  list  as  those  that  are  most  apt  to  find  their 
way  into  the  preparations  of  the  microscopist:  Oil  globules; 
milk;  starch  from  the  potato,  wheat  and  rice;  bread  crumbs; 
feathers;  worsted;  fibres  of  flax,  cotton  and  silk  of  different 
colors;  human  hair,  cat's  hair  and  hair  from  blankets;  the  scales 
of  butterflies  and  moths,  particularly  those  from  the  common 
clothes  moth;  fibres  of  wood,  fragments  of  tea  leaves,  hairs 
from  plants,  vegetable  cellular  tissue  and  spiral  vessels;  particles 
of  sand.  The  curious  circumstances  under  which  such  bodies 
will  find  their  way  into  a  specimen  was  recently  illustrated  in  the 
author's  experience.  In  a  liquid  submitted  for  examination, 
and  said  to  be  pure,  he  found  foreign  matter.  It  proved  to  be 
brick  dust,  used  to  clean  the  tin  funnel  with  which  the  vessel 
was  filled,  and  which  had  been  washed  in  by  the  passage  of 
the  fluid.  The  student  can  have  no  better  exercise  than  to 
examine  these  intruders  and  familiarize  himself  with  their,  ap- 
pearance* 

PreserTative  Processes.— The  object  of  all  preserva- 
tive processes  is  to  prevent  any  change  either  in  the  structure 
or  composition  of  the  object.  An  object  may  be  most  perfectly 
prepared  and  beautifully  mounted,  but  if  it  be  not  so  treated 
as  to  preserve  it  from  change,  the  labor  thus  expended  is  wasted, 
as  regards  the  preservation  of  a  permanent  record.  And  yet 
how  many  objects  there  are  that  we  would  like  to  keep  for 
future  examination  and  comparison,  or  to  show  to  friends. 
This  department  of  the  treatment  of  objects  is,  therefore,  of 
great  importance,  and  success  in  it  can  only  be  obtained 
through  a  thorough  understanding  of  the  principles  involved. 

There  are  four  methods  in  common  use  for  the  preservation 
of  perishable  animal  and  vegetable  substances:  1,  Constant  ex- 
posure to  temperature  considerably  below  the  freezing  point  of 
water;  2,  the  perfect  exclusion  of  air;  3,  reduction  to  a  state  of 
complete  dryness;  and  4,  the  employment  of  certain  anti-septic 


V  Oi1  THE   MICKOSCOPE.  199 

compound.  The  third  and  fourth  are  the  methods  usually  em- 
ployed in  microscopy,  but  the  same  principles  which  render  the 
second  method  so  successful  in  the  preservation  of  canned  fruits 
and  meats,  deserve  the  attention  of  the  microscopist. 

Drying,  as  a  preservative  process,  can  be  applit d  to  but  few 
specimens,  chiefly  transparent  insect  preparations,  and  opaque 
objects.  Blood  and  similar  matters  are  also  sometimes  pre- 
served by  drying.  Such  preparations  are  so  easily  dried  that 
no  special  directions  are  needed.  Warming  them  over  a  lamp, 
or  preferably  on  a  water-bath,  before  applying  the  thin  glass 
cover  (as  directed  in .  the  section  on  mounting  objects)  is  al- 
most always  sufficient.  Where  the  specimen  is  liable  to  be  in- 
jured by  heat  it  may  be  dried  by  placing  it  over  sulphuric  acid, 
and  covering  both  acid  and  preparation  with  a  bell  jar  having 
ground  edges  and  resting  on  a  perfectly  flat  plate  of  glass.  The 
acid  soon  absorbs  all  the  moisture  and  renders  the  object  perfectly 
dry.  Where  a  cell  is  used  for  an  opaque  object,  and  dry  ness  is 
essential,  great  care  must  be  taken  to  make  the  cell  impervious 
to  air,  otherwise  dampness  will  be  sure  to  penetrate,  and  if  the 
object  be-  of  animal  or  vegetable  origin,  fungi  will  be  very  apt  to 
grow  on  it.  We  have  found  cells  of  cardboard  peculiarly 
liable  to  this  defect,  and  such  cells  should  always  be  thoroughly 
saturated,  and  coated  with  varnish,  such  as  gold  size  or  Canada 
balsam. 

The  great  dependence  of  the  microscopist,  however,  is  in  the 
employment  of  certain  preservative  media,  of  the  most  impor- 
tant of  which,  the  following  is  a  list: 

CANADA  BALSAM. — Of  all  the  media  employed  for  the  mount- 
ing and  preservation  of  objects,  Canada  balsam  is  undoubtedly 
the  most  generally  useful,  and  it  is  probable  that  more  objects  are 
mounted  in  this  material  than  in  all  the  other  media  put  together. 
As  a  preservative  it  is  perfect,  and  its  action  in  rendering  many 
objects  transparent  and  clear  is  often  of  great  value.  Frey  tells 
us  that  "several  sorts  of  Canada  balsam  occur  in  commerce. 
To  be  good  it  should  be  of  thick  consistence,"  nearly  colorless, 
and  thoroughly  transparent."  One  difficulty,  however,  is  that 
much  of  the  Canada  balsam  that  is  sold  is  factitious,  being 
made  of  cheap  resins  dissolved  in  impure  turpentine.  Such 


200  SELECTION  AND    USii 

balsam  soon  becomes  cloudy,  and  is  very  apt  to  crack.  Balsam 
that  is  too  highly  colored  may  be  bleached  by  exposure  to  sun- 
light— a  process  applied  by  most  opticians  to  the  balsam  used 
by  them  for  cementing  the  lenses  of  achromatic  combinations. 
Balsam  when  new  is  quite  fluid,  too  much  so,  indeed,  for  the 
mounting  of  most  objects.  On  the  other  hand,  old  balsam  is 
thick,  and  is  apt  to  crack.  Microscopists  generally  keep  balsam 
in  wide-mouthed  bottles,  and  take  out  what  is  wanted  by  means 
of  a  glass  rod.  As  the  process  of  evaporation,  which  makes 
balsam  thick  and  viscid,  goes  on  more  slowly  in  narrow-mouthed 
bottles,  we  prefer  the  latter,  and  transfer  the  balsam  to  the  glass 
slide  by  means  of  a  fine  wire  with  a  small  loop  at  the  end.  The 
wire  is  passed  through  a  cork,  or  preferably  a  wooden  stopper, 
and  descends  to  such  a  depth  as  to  be  just  below  the  surface  of 
the  balsam.  As  the  latter  is-  used  up,  the  wire  is  pushed  down, 
and  if  cemented  in  its  place  by  the  balsam,  a  little  heat  soon  frees 
it.  The  latter  remark  applies  also  to  the  wooden  stopper,  which 
is  very  apt  to  stick  in  the  neck  of  the  bottle.  A  very  slight  ex- 
posure to  the  flame  of  a  spirit  lamp  is  sufficient  to  loosen  it. 

SOLUTION  or  BALSAM. — When  the  objects  that  are  to  be  pre- 
served in  balsam  would  be  injured  by  the  heat  necessary  to 
melt  it,  it  is  advisable  to  use  a  solution  of  balsam  in  ether  or 
chloroform.  The  balsam  used  for  making  the  solution  should  be 
old  and  thick.  This  solution  is  frequently  sold  with  the  label, 
"  Balsam  for  use  without  heat." 

COLOPHONY. — Thiersch  recommends  a  solution  of  resin  or 
colophony  in  absolute  alcohol.  The  advantage  which  this  ma- 
terial presents  is  that  the  preparation  may  be  placed  in  it  di- 
rectly from  the  absolute  alcohol,  without  becoming  cloudy, 
and  without  prejudice  to  the  durability  of  the  specimen.  He 
advises  the  microscopist  to  prepare  the  colophony  himself  from 
Venice  turpentine,  which  is  done  by  dissolving  it  in  an  equal 
volume  of  ether,  filtering  it  through  paper,  and  evaporating, 
until,  when  cold,  it  breaks  with  a  conchoidal  fracture.  The 
material  that  remains  is  then  to  be  dissolved  in  absolute  alco- 
hol until  it  is  of  a  syrupy  consistence. 


OF    THE   MICROSCOPE.  201 

DAMAB  MEDIUM. — Gum  damar  has  been  recently  introduced 
amongst  the  materials  used  by  microscopists,  and  with  some  ii> 
has  found  great  favor.  Carpenter  speaks  highly  of  it.  Dia- 
toms are  said  to  show  better  in  it  than  in  balsam,  and  for 
delicate  physiological  preparations,  especially  transparent  in- 
jections, it  is  very  excellent.  It  is  thus  prepared:  Half  an 
ounce  of  gum  damar  is  dissolved  in  one  ounce  of  oil  of  turpen- 
tine, and  half  an  ounce  of  gum  mastic  in  two  ounces  of  chloro- 
form. The  solutions  are  filtered  and  mixed. 

Ordinary  damar  varnish,  such  as  is  used  by  painters,  is  some- 
limes  sold  for  microscopical  purposes,  but  it  does  not  give 
satisfactory  results. 

Preparations  which  have  been  preserved  and  mounted  in 
balsam  or  damar  are  very  durable,  while  those  that  are 
mounted  in  fluids  are  a  source  of  continual  annoyance  and  loss. 

Many  microscopists,  therefore,  exclude  from  their  cabinets 
all  preparations  mounted  in  liquid  on  the  ground  that  sooner 
or  later  they  will  become  worthless.  And  many  of  our  best 
dealers  refuse  to  have  anything  to  do  with  them.  Neverthe- 
less, as  Frey  well  says,  "  the  natural  condition  of  the  tissues  is 
completely  represented  only  when  mounted  in  a  moist  condi- 
tion. This  method  permits  of  the  most  accurate  recognition  of 
delicate  textural  relations,  pale  cells  and  fibres,  etc.,  and  should 
not  be  omitted  with  any  tissue  in  the  production  of  histologi- 
cal  collections." 

GLYCERINE. — At  the  head  of  the  list  of  preservative  media 
for  moist  preparations  stands  glycerine.  "Its  strong  refrac- 
tive power,  its  property  of  combining  with  water,  and  of  at- 
tracting the  same  from  the  atmosphere,  render  it  an  invalua- 
ble medium  for  mounting  animal  tissues  containing  water. 
It  may  be  truly  said,  that  what  Canada  balsam  is  to  dry  tissues, 
glycerine  is  to  moist  ones." — (Frey.)  Much  of  the  glycerine  in 
market  is  very  impure,  and  although  the  impurities  do  not 
show  themselves  very  strongly  at  first,  they  soon  become  mani- 
fest by  the  darkening  of  the  liquid,  (owing  probably  to  the 
presence  of  lead),  ?.nd  the  formation  of  a  cloudy  precipitate. 
Dr.  Beale  strongly  recommends  Price's  glycerine,  and  we  have 
found  it  very  excellent. 


202  SELECTION   AND    USE 

When  employed  as  a  preservative,  glycerine  is  used  either 
pure  or  diluted,  according  to  circumstances.  Equal  parts  of 
glycerine  and  water  form  a  very  excellent  medium  for  most 
objects.  It  is  alleged,  however,  that  fungi  are  very  apt  to  grow 
in  glycerine  and  its  solutions.  We  are  inclined  to  believe 
that  this  may  be  avoided  by  adopting  the  precaution  detailed 
at  the  end  of  this  section.  We  have  now  before  us  specimens 
that  were  mounted  in  pure  glycerine  and  water,  eighteen  years 
ago,  and  they  are  still  quite  perfect.  If,  however,  there  should 
be  any  danger  in  this  direction,  the  addition  of  a  little  camphor 
will  prevent  the  evil.  Glycerine  exerts  a  powerfully  solvent 
action  on  many  salts,  particularly  salts  of  lime,  such  as  the  car- 
bonate, and  hence  it  is  employed  for  preventing  scale  in  the 
boilers  of  steam-en gines.  This  property  renders  it  dangerous 
to  use  it  for  the  preservation  of  structures  containing  com- 
pounds of  lime. 

GLYCERINE  JELLY. — The  original  directions  given  by  Law- 
ranee  are  as  follows:  "  Take  any  quantity  of  Nelson's  gelatine, 
(any  good  gelatine  will  answer,  however,)  and  let  it  soak  for 
two .  or  three  hours  in  cold  water;  pour  off  the  superfluous 
water,  and  heat  the  soaked  gelatine  until  melted.  To  each 
fluid  ounce  of  the  gelatine  add  one  drachm  of  alcohol,  and  mix 
well;  then  add  a  fluid  drachm  of  the  white  of  an  egg.  Mix 
well  while  the  gelatine  is  fluid  but  cool.  Now  boil  until  the 
albumen  coagulates,  and  the  gelatine  is  quite  clear.  Filter 
through  fine  flannel,  and  to  each  fluid  ounce  of  the  clarified 
gelatine  add  six  fluid  drachms  of  Price's  pure  glycerine,  and 
mix  well.  For  the  six  fluid  drachms  of  glycerine  a  mixture  of 
two  parts  of  glycerine  to  four  of  camphor  water  may  be  sub- 
stituted." 

Glycerine  jelly  is  a  very  excellent  medium,  and  is  easily  used. 
At  ordinary  temperatures  it  is  quite  solid,  but  when  slightly 
heated  it  melts,  and  may  be  used  like  balsam,  directions  for 
mounting  in  which  will  be  found  in  the  next  section.  Objects 
that  are  to  be  mounted  in  glycerine  jelly  should  be  soaked 
until  thoroughly  saturated  with  a  mixture  of  7  parts  glycerine, 
6  parts  water,  and  1  part  alcohol.  It  is  also  well,  after  immers- 
ing them  in  the  melted  jelly,  to  place  the  slide  for  a  short  time 


OF    THE    MICROSCOPE.  203 

on  a  water  bath  heated  to  about  125°  Fah.     The  jelly  then  pen- 
etrates every  part  of  the  preparation. 

When  intended  for  use  in  very  warm  climates  the  proportion 
of  the  gelatine  to  the  other  ingredients  should  be  increased. 

HANTZSCH'S  FLUID.  —Very  beautiful  preparations  of  delicate 
vegetable  forms  have  been  prepared  with  this  liqui.l,  even  the 
coloring  matter  being  left  unaltered.  It  consists  of  3  parts  of 
pure  alcohol,  2  parts  of  distilled  water  and  one  part  of  glycer- 
ine. The  object,  placed  in  a  cell,  is  covered  with  a  drop  of 
this  liquid,  and  then  set  aside  under  a  bell-glass.  The  alcohol 
and  water  soon  evaporate,  so  that  the  glycerine  alone  is  left, 
and  another  drop  of  the  liquid  is  then  to  be  added,  and  a 
second  evaporation  permitted;  the  process  being  repeated  if 
necessary,  until  enough  glycerine  is  left  to  fill  the  cell, 
which  is  then  to  be  covered  and  closed  in  the  usual  macner. 
We  have  used  this  liquid  with  gratifying  success.  It  is  easily 
prepared,  is  not  difficult  to  use,  and  it  gives  very  excellent  re- 
sults. 

GLYCERINE  AND  GUM. — Of  this  medium  Carpenter  says:  "  For 
many  objects  that  would  be  injured  by  the  small  amount  of  heat 
required  to  melt  Deane's  gelatine  or  glycerine  jelly,  the  gly- 
cerine and  gum  medium  of  "Mr.  Farrants  will  be  found  very 
useful.  This  is  made  by  dissolving  4  parts  by  weight  of  picked' 
gum  arabic  in  4  parts  of  cold  distilled  water,  and  adding  2  parts 
of  glycerine.  The  solution  must  be  made  without  the  aid  of 
heat,  the  mixture  being  occasionally  stirred,  but  not  shaken, 
whilst  it  is  proceeding:  after  it  has  been  completed,  the  liquid 
should  be  strained  (if  not  perfectly  free  from  impurity)  through 
fine  cambric  previously  well  washed  out  by  a  current  of  clear 
cold  water;  and  it  should  be  kept  in  a  bottle  closed  with  a  glass 
stopper  or  cap  (not  with  cork),  containing  a  small  piece  of 
camphor.  The  great  advantage  of  this  medium  is  that  it  can 
be  used  cold,  and  yet  soon  viscifies  without  cracking;  it  is  well 
suited  to  preserve  delicate  animal  as  well  as  vegetable  tissues, 
and  in  most  cases  it  increases  their  transparency. 

DEANE'S  GELATINE. — Before  the  introduction  of  glycerine 
jelly,  Deane's  gelatine  was  a  favorite  medium,  and  we  still  use 


204  SELECTION  AND    USE 

it  with  success.  Take  gelatine,  1  ounce;  honey,  5  ounces; 
water,  5  ounces;  rectified  spirit,  £  ounce;  creosote,  6. drops. 
Soak  the  gelatine  in  water  until  soft,  and  then  add  it  to  the 
honey,  which  has  been  previously  raised  to  a  boiling  heat  in 
another  vessel.  Then  boil  the  mixture,  and  when  it  has  cooled 
somewhat  add  the  creosote  mixed  with  the  spirit.  Lastly,  ni- 
ter through  fine  flannel.  When  required  for  use,  the  bottle 
containing  the  mixture  must  be  slightly  warmed,  and  a  drop 
placed  on  the  preparation  upon  the  glass  slide,  which  should 
also  be  warmed  a  little.  Next,  the  glass  cover,  after  having 
been  breathed  upon,  is  to  be  laid  on  with  the  usual  precau- 
tions. The  edges  may  be  covered  with  a  coating  of  Bruns- 
wick black.  Care  must  be  taken  that  the  surface  of  the 
drop  does  'not  become  dry  before  the  application  of  the 
glass  cover;  and  the  inclusion  of  air-bubbles  must  be  carefully 
avoided. 

ALCOHOL. — Mixed  with  water  in  various  proportions,  alcohol 
forms  one  of  our  best  preservative  liquids,  for  both  animal  and 
vegetable  substances.  The  chief  objection  to  it  is  the  difficulty 
with  which  it  is  retained  in  the  cell. 

THWAITE'S  FLUID. — Take  water,  16  ounces;  alcohol,  1  ounce; 
creosote,  sufficient  to  saturate  the  spirit;  chalk,  as  much  as  may 
be  necessary.  Mix  the  creosote  and  spirit,  stir  in  the  chalk 
with  the  aid  of  a  pestle  and 'mortar,  and  let  the  water  be  added 
gradually.  Next  add  an  equal  quantity  of  water  saturated  with 
camphor.  Allow  the  mixture  to  stand  for  a  few  days  and  filter. 
Used  for  preserving  desmidise,  and  also  animal  substances. 

BEALE'S  LIQUID. — Creosote,  3  drachms;  wood  naphtha,  6 
ounces;  distilled  water,  64  ounces;  chalk,  as  much  as  necessary. 
Mix  the  naphtha  and  creosote,  then  add  as  much  prepared 
chalk  as  may  be  sufficient  to  form  a  thick,  smooth  paste;  after- 
wards add,  very  gradually,  a  small  quantity  of  the  water,  which 
must  be  well  mixed  with  the  otjer  ingredients  in  a  mortar. 
Add  two  or  three  small  lumps  of  camphor,  and  allow  the  mix- 
ture to  stand  in  a  lightly  covered  vessel  for  a  fortnight  or  three 
weeks  with  occasional  stirring.  The  almost  clear  supernatant 
fluid  may  then  be  poured  off  and  filtered  if  necessary.  It  should 
be  kept  in  well-corked  or  stoppered  bottles. 


Oi1  THE  MICROSCOPE.  205 

GOADBY'S  FLUIDS. — Goadby  used  two  distinct  fluids,  desig- 
nated by  letters  A  and  B,  the  difference  being  that  alum  was  a 
constituent  of  one  and  not  of  the  other.  Of  both  fluids  there 
were  several  degrees  of  strength,  which  were  designated  by 
numbers.  A  fluid,  as  usually  employed  (A2),  consisted  of  rock 
salt,  4  ounces;  alum,  2  ounces;  corrosive  sublimate,  4  grains; 
boiling  water,  2  u  ,-ts.  To  make  the  B  fluid  take  rock  salt,  8 
ounces;  corrosixv  sublimate,  2  grains;  boiling  water,  1  quart 

PACKI'S  FLUID. — Take  corrosive  sublimate,  1  part;  pure 
ohloride  of  sodium  (common  salt),  2  parts;  glycerine,  13  parts; 
distilled  water,  113  parts.  This  mixture  is  allowed  to  stand  for 
jit  least  two  months.  After  that  time  it  is  prepared  for  use  by 
mixing  one  part  of  it  with  three  parts  of  distilled  water,  and 
filtering  it  through  filtering  paper.  This  fluid  is  very  strongly 
recommended  by  Frey.  It  is  used  for  blood  globules,  nerves 
and  ganglia,  the  retina,  cancer  cells,  and  especially  delicate  pro- 
teinous  tissues. 

CASTOR  OIL. — This  is  used  for  preserving  certain  crystals. 
The  best  cold-drawn  castor  oil  answers  the  purpose. 

There  are  a  few  general  rules  which  we  have  found  essential 
to  the  successful  use  of  these  media,  but  which  ere  often 
neglected,  the  result  being  the  ultimate  destruction  of  the 
specimens.  One  of  the  most  important  points  is  the  use  of  an 
abundance  of  the  medium  (we  are  now  talking  of  preserving, 
not  mounting]  and  the  gradual  saturation  of  the  object  with  it. 
A  piece  of  fresh  muscle,  simply  mounted  in  a  shallow  cell  with 
a  drop  or  two  of  Goadby's  fluid,  will  spoil  in  a  very  short  time. 
The  same  object,  properly  treated,  may  be  preserved  indefin- 
itely. The  proper  course  is  to  completely  immerse  the  object 
in  a  considerable  quantity  of  the  liquid,  and  if  necessary 
change  the  liquid  several  times  until  the  substance  to  be  pre- 
served has  been  thoroughly  subjected  to  the  action  of  the 
medium.  For  this  purpose  the  quantity  contained  in  ordinary 
cells  is  altogether  too  little;  small  cups,  basins,  large  watch- 
glasses,  etc.,  are  needed.  It  must  be  remembered  that  the  sub- 
stance acted  upon  generally  absorbs  certain  constituents  of  the 
preserving  fluid,  and  hence  the  latter  is  left  either  very  weak 


206  SELECTION    AND    USE 

or  there  is  an  unequal  distribution  of  the  constituents  as  re- 
gards the  substance  itself  and  the  surrounding  fluid.  Moreover 
the  fluids  contained  in  many  objejts  are  displaced  by  the  pre- 
serving medium,  and  tend  to  dilute  the  latter.  In  most  cases, 
therefore,  where  the  preserving  medium  is  a  liquid,  the  desired 
result  is  best  attained  by  soaking  the  substance  in  the  fluid  for 
several  days  before  mounting,  changing  the  liquid  two  or  three 
times,  and  finally  mounting  in  fresh  fluid  of  regular  strength. 
"We  would  lay  great  stress  upon  this  point,  having  seen  many 
fine  preparations  spoiled  by  pursuing  a  different  course.  The 
late  Dr.  Goadby,  whose  skill  in  this  department  was  well 
known,  always  insisted  upon  this  course,  and  during  a  some- 
what extended  intercourse  with  him,  and  observation  of  his 
methods  and  processes,  we  became  fully  convinced  of  its  im- 
portance. 

"With  many  preservative  liquids,  it  is  well  to  begin  with  a 
diluted  article,  and  gradually  increase  the  strength  at  each 
change  of  fluid  until  the  proper  strength  has  been  reached. 
This  course  is  specially  recommended  with  glycerine  and  saline 
solutions. 

Another  point  which  demands  attention  is  the  entire  exclu- 
sion of  air,  especially  of  oxygen.  Now  air  adheres  with  great 
tenacity  to  most  surfaces,  such  as  those  of  glass  or  metal,  and  it 
dissolves  to  a  considerable  extent  in  all  watery  solutions.  To 
get  rid  of  it,  the  surface  of  the  cell  and  cover  should  be  either 
well  warmed,  and  then  allowed  to  cool  just  before  being  filled, 
or  washed  with  alcohol  (after  which  it  may  be  dried).  To  ex- 
pel the  air  from  the  liquids,  they  should  be  boiled,  and  to  pre- 
vent the  absorbtion  of  a  fresh  dose  of  air,  they  should  be  kept 
well  stoppere  d.  But  as  air  will  find  access  to  the  liquids  so  as 
ultimately  to  saturate  them,  it  is  necessary  to  boil  the  fluids  at 
frequent  intervals,  so  as  to  get  rid  of  this  element.  Without 
strict  attention  to  these  points  it  is  almost  impossible  to  pre- 
serve animal  substances  for  any  length, of  time  in  saline  fluids. 

Mounting  Objects.— For  the  purpose  of  conveniently 
exhibiting  and  comparing  objects,  and  arranging  them  in  cab- 
inets where  they  can  be  at  all  times  accessible,  it  is  necessary  to 
mount  them  securely  in  such  a  manner  that  they  may  be  easily 


OF  THE  inCBOSCOPE.  207 

handled.  For  purposes  of  mere  examination  and  study, 
mounting  is  unnecessary,  but  when  the  objects  are  to  be  kept 
for  future  reference  it  is  indispensable.  It  is  true  that  where 
the  specimens  are  large  they  might  be  kept  in  bottles  in  a  pre- 
servative fluid,  and  taken  out  when  wanted.  This  would  be 
very  inconvenient,  however,  and  with  very  minute  or  delicate 
objects  it  would  be  almost  impracticable. 

There  are  three  modes  in  which  objects  are  mounted:  1.  Dry, 
the  object  being  simply  attached  to  the  slide  and  suitably  pro- 
tected. 2.  In  balsam,  the  object  being  immersed  in  Canada 
balsam,  clamar  medium,  copal  varnish,  or  some  similar  mate- 
rial. 3.  In  fluid,  the  object  being  mounted  in  some  of  the  pre- 
servative liquids  previously  described.  Specimens  may  be 
mounted  in  any  of  these  ways,  so  as  to  be  viewed  either  as 
transparent  or  opaque  objects,  and  the  instruments  and  mate- 
rials required  are  neither  numerous  nor  expensive.  "W^th  those 
named  in  the  following  list  almost  any  ordinary  object  may  be 
neatly  put  up,  though  it  is  of  course  to  be  expected  that  occa- 
sions will  frequently  arise  when  special  instruments  and 
methods,  which  are  not  described  by  any  author,  will  be 
needed.  Experience  alone  can  enable  the  microscopist  to  treat 
such  cases  successfully. 

SLIDES. — Most  objects  are  mounted  between  two  pieces  of 
glass,  one  of  which  is  called  the  slide  and  the  other  the  cover. 
As  it  is  convenient  to  have  these  slides  all  the  same  size,  so  that 
they  may  be  easily  arranged  in  cabinets,  the  Microscopica  1 
Society  of  London  has  adopted  a  slide  three  inches  long  by  one 
inch  wide  as  the  standard  size  for  use  amongst  their  members, 
and  this  size  has  been  generally  adopted  by  microscopists 
throughout  the  world.  AU  the  best  slides  that  are  found  in 
market  are  of  this  size,  and  the  microscopist  who  fails  to  adopt 
it  will  be  subject  to  great  inconvenience  when  he  desires  to  ex- 
change objects  with  others  who  are  pursuing  similar  studies. 
Several  other  sizes  are  employed  by  the  French,  most  of  them 
being  quite  small  (2f  by  f  and  2J  by  £),  but  as  these  small  slides 
are  the  only  ones  that  can  be  used  with-some  French  micro- 
scopes— the  stages  of  which  are  too  small  to  take  a  slide  3  by  1 — 
they  are  usually  kept  in  stock  by  dealers  in  microscopic  appar- 


208  SELECTION   AND   TTSE 

utus.  Small  slides  have  this  advantage,  that  they  cost  less,  and 
take  up  less  room  in  a  cabinet.  Large  slides  look  best,  and  af- 
ford more  room  for  descriptive  labels,  which  is  an  important 
point.  But  since  slides  3  by  1  have  been  adopted  by  common 
consent,  the  mtcroscopist  who  mounts  specimens,  or  who  buys 
objects  mounted  on  slides  of  a  different  size,  commits  a  mis- 
take for  which  the  advantages  offered  by  the  small  slides  are 
but  a  slight  compensation.  The  only  exceptions  to  this  rule 
are  where  the  objects  are  too  large  to  be  mounted  securely  on  a 
slide  of  standard  size,  or  where  a  large  number  are  to  be  pre- 
pared for  the  purpose  of  illustrating  some  special  series  of  in- 
vestigations. It  is  to  be  presumed  that  such  a  series  will  never 
be  broken  up  and  separated,  and  as  it  will  in  all  probability  be 
assigned  to  its  own  cabinet,  it  is  sometimes  of  advantage  to 
have  it  upon  slides  of  a  size  other  than  that  in  common  use. 
As  the  objects  composing  such  a  series  will  probably  be  num- 
bered and  catalogued,  there  is  no  necessity  for  extended  de- 
scriptions on  the  labels,  and  therefore  slides  of  half  the  usual 
size  (l£  by  1)  will  serve  very  well.  The  cabinet  may  thus  be 
reduced  in  bulk  by  one-half.  We  have  a  special  cabinet,  illus- 
trative of  textile  fibres,  mounted  upon  slides  of  small  size3  and 
find  it  quite  convenient. 

The  glass  from  which  slides  are  cut  should  be  free  from  air- 
bubbles,  scratches  and  that  wavy  appearance  .which  is  due 
either  to  inequalities  in  the  surface  or  to  irregularities  in  the 
composition  of  the  glass  itself.  Ordinary  window  glass  is  en- 
tirely unfit  for  the  purpose.  The  most  suitable  kind  is  plate 
glass,  the  surface  of  which  has  been  ground  and  polished,  so  as 
to  be  perfectly  even  and  smooth.  Glass  of  this  kind  is  used  for 
looking-glasses  and  by  photographers,  and  when  other  material, 
could  not  be  had,  we  have  made  very  excellent  slides  out  of 
broken  looking-glasses  and  photographer's  plates,  though  it  is 
difficult  to  get  the  latter  thin  enough.  Slides  of  good  glass 
are,  however,  manufactured  in  quantity  and  sold  at  a  reason- 
able price,  so  that  under  ordinary  circumstances  it  will  hardly 
pay  the  microscopist  to  cut  out  his  own  slides.  Moreover  the 
slides  sold  by  the  dealers  have  the  edges  neatly  ground,  an 
operation  which  the  microscopist  will  find  tedious  and  trouble- 
some. 


OF  TSU  MICROSCOPE.  '200 

» 

As  procured  from  the  manufacturers,  tlic  slides  are  always 
dirty,  never  having  been  washed  after  the  process  of  grinding 
and  polishing  the  edges.  If  this  dirt  were  soft  it  would  not 
matter  so  much,  but  it  is  in  general  hard  and  gritty — being  in 
fact  the  grinding  sand — and  the  consequence  is  that  the  surfaces 
of  the  slides  are  very  apt  to  be  scratched  and  injured.  There 
is  but  one  firm  that  exports  slides  to  this  country,  and  they  are 
very  careless  in  this  respect.  Out  of  a  gross  of  slides  it  is  often 
difficult  to  find  two  dozen  that  are  not  so  scratched  as  to  be 
worthless  for  the  finest  class  of  work.  Having  procured  the 
slides,  however,  the  first  thing  to  do  is  to  c'.jaii  and  assort 
them.  They  should  be  cleaned  by  being  rinsed  in  water  con- 
taining a  little  washing  soda;  the  dirt  being  removed  if  neces- 
sary by  the  use  of  an  old  nail  brush  or  tooth  brush.  Until 
this  has  been  done  they  should  not  be  wiped  with  cloth  or 
leather,  for  by  so  doing  the  particles  of  sand  are  dragged  along 
the  surface,  making  a  deep  mark.  They  should  then  be  washed 
in  pure  water,  carefully  wiped  with  a  soft  cloth,  and  assorted 
for  thickness  and  quality.  It  is  in  general  best  to  sort  them 
into  three  classes — thick,  medium  and  thin — the  latter  being 
used  for  test  and  other  very  delicate  objects.  Elaborate  instru- 
ments have  been  devised  for  measuring  the  thickness  of  the 
slides,  so  as  to  assort  them  accurately,  but  they  are  entirely  un- 
necessary; the  eye  is  a  sufficiently  accurate  guide.  To  deter- 
mine tbeir  quality,  they  must  be  examined  under  the  micro- 
scope, and  as  it  is  only  the  central  portion  that  is  of  any  con- 
sequence in  this  case,  we  place  them  on  a  brass  plate,  3  by  1, 
with  the  edges  slightly  turned  up,  and  having  a  hole  five- 
eighths  of  an  inch  in  diameter  in  the  centre.  That  part  which 
lies  over  the  hole  is  the  only  part  which  it  is  necessary  to  ex- 
amine. Slides  which  contain  air-bubbles,  striae  or  scratches, 
are  at  once  laid  aside  to  be  used  either  for  opaque  objects  or 
those  of  a  very  coarse  kind.  Those  that  are  perfect  are  care- 
fully stored  away  where  they  will  not  be  subject  to  injury. 

COVERS. — After  being  properly  arranged  on  the  slide  with  a 
suitable  preservative  medium,  the  objects  must  be  covered  with 
a  small  piece  of  thin  glass.  Glass  intended  specially  for  this 
purpose  is  made  in  England,  and  imported  either  in  sheets  or 


210  SELECTION  AND    USE 

cut  into  squares  and  circles  of  suitable  sizes.  Directions  for 
cutting  these  covers  would  be  out  of  place  here.  The  beginner 
•will  always  find  it  most  economical  to  buy  them  ready  cut.  Of 
the  two  kinds — round  and  square — the  former  are,  for  all  ordi- 
nary purposes,  the  most  convenient,  as  covers  of  this  shape  are 
best  suited  to  cells  made  with  the  turn-table,  and  they  may  also 
be  finished  more  easily  and  neatly  than  the  square  ones. 

Covers  should  be  carefully  assorted  for  thickness,  since  the 
thickness  of  the  cover  exerts  a  material  influence  on  the  per- 
formance of  all  lenses  except  those  of  the  lowest  power  or 
quality.  Where  objectives  which  do  not  adjust  for  thickness  of 
cover  are  employed,  the  microscopist  should  find  out  the  exact 
thickness  to  which  they  have  been  corrected  by  the  maker,  and 
use  glass  of  this  thickness  in  covering  all  objects  that  are  to  be 
examined  by  means  of  these  lenses. 

The  inexperienced  student  will  be  apt  to  find  some  difficulty 
in  cleaning  these  covers.  They  are  so  fragile  that  it  is  difficult 
to  rub  them,  so  as  to  remove  dirt,  without  breaking  them. 
The  best  method  is  to  soak  them  in  a  weak  solution  of  potash, 
rinse  them  off  carefully  several  times  with  clean  water,  and  after 
pouring  the  last  water  off,  give  them  a  final  rinsing  by  taking 
them  up  in  a  pair  of  forceps  and  moving  them  about  in  a  tum- 
bler of  clean  water.  They  should  then  be  laid  (singly,  of 
course)  on  a  wiping  block  and  wiped.  Wiping  blocks  are  made 
by  covering  a  flat  block  of  wood  with  chamois  leather  or  linen, 
drawn  tightly  so  as  to  present  a  flat  but  somewhat  soft  surface. 
These  blocks  are  generally  made  round  and  with  handles,  but  we 
prefer  them  oblong  (4  by  1^  inches)  and  'without  handles.  One 
of  them  is  laid  on  the  table  face  up;  upon  this  face  the  thin 
glass  is  laid  and  wiped  with  the  other  block.  In  this  way  the 
thinnest  glass  can  be  cleaned  without  risk  of  fracture. 

CELLS— TURN-TABLE. — All  objects  that  are  mounted  dry  or  in 
fluid  should  be  placed  in  cells,  as  unless  this  is  done  it  is  dif- 
ficult to  arrange  the  object  properly  or  to  secure  the  thin  cover 
permanently.  In  the  majority  of  cases  these  cells  consist  of 
little  more  than  a  ring  of  cement  laid  on  the  glass  slide  and 
allowed  to  harden,  and  their  depth  does  not  exceed  the  thick- 
ness of  a  sheet  of  paper.  Such  cells  are  in  constant  demand, 


OP  THE  MICROSCOPE. 

and  are  almost  always  made  by  the  microscopist  himself  by 
means  of  a  little  instrument  known  as  a  turn-table  or  whirling 
table,  of  which  there  are  several  different  forms  in  market.  A 
cheap  and  efficient  form  is  shown  in  Fig.  62.  The  table  is  sup- 
ported by  a  spindle  upon  which  it  turns,  motion  being  com- 
municated by  means  of  a  milled  ring.  The  slide  is  held  in  its 
place  by  two  spring  clips,  and  it  is  brought  to  the  centre  by 
means  of  a  guide  or  bar,  c,  with  a  square  projection.  This  is 
carefully  arranged,  so  that  a  slide  3  by  1  shall  be  accurately 
centered.  Hence  it  follows  that  the  rings  and  cells  on  all  the 
slides  put  up  by  the  owner  may  be  instantly  and  accurately 
t. 


Fig.   62. — TURN-TABLE. 

centered  by  simply  placing  them  on  the  table  and  bringing 
them  up  to  a  firm  bearing  against  the  guide.  This  bar  01 
guide  may,  however,  be  instantly  removed  when  desired,  and 
when  this  is  done,  any  cell  may  be  truly  centered  by  the  usual 
methods.  This  turn-table,  therefore,  enables  us  always  to  bring 
cells  of  our  own  make  instantly  to  a  perfectly  accurate  centre, 
while  other  cells  can  be  centered  at  any  time  with  very  little 
trouble0 

To  most  turn-tables  there  lies  the  objection  that  the  devices 
for  centering  and  holding  the  slide  make  one  side  heavier  than 
the  other,  and  consequently,  as  every  mechanic  knows,  irregu- 
lar and  eccentric  motion  is  the  result.  On  many  otherwise 
well-made  instruments  it  is,  from  this  cause,  impossible  to  make 
a  true  cell,  particularly  if  we  attempt  to  work  at  a  high  speed. 


212 


SELECTION1  AND    tJSB 


In  the  turn-table  just  described,  provision  is  made  to  obviate 
this  difficulty.  A  heavy-headed  screw,  of  the  precise  weight 
necessary,  is  screwed  into  the'  under  surface  of  the  table,  and 
gives  a  perfect  balance  to  the  wheel.  It  then  runs  smoothly 
and  truly. 

Numerous  attempts  have  been  made  to  produce  a  self-center- 
ing table,  i.  e.t  one  in  which  the 
slides  would  be  truly  centered 
without  requiring  care  and  skill 
on  the  part  of  the  operator.  One 
of  the  earliest  forms  was  that  of 
Dr.  Matthews,  the  centering  part 
of  which  is  shown  in  Fig.  t>3. 
Upon  the  surface  of  the  table  he 
arranges  two  triangular  plates 
of  brass,  which  rotate  upon  pins 
placed  at  equal  distances  on  each 
side  of  the  centre,  and  as  the 
plates  are  of  the  same  size,  when- 
Fig.  63. — MATTHEW'S  TUBS-TABLE,     ever  their  inner  faces  are  par- 
allel, these  faces  must  be  equi- 
distant from  the  centre.     Hence,   when  a  slide  with  parallel 
sides  is  placed  between  them,  and  the  plates  turned  so  as  to 
press  upon  the  sides  of  the  slide,  the  slide  will  be  truly  cen- 
tered so  far  as  its  width  is  concerned.     It  is  centered  for  length 
by  a  stationary  pin,  against  which  the  end  is  always  brought. 
Slides  of  irregular  size  are  therefore  centered  only  one  way. 

There  are  at  present,  however,  before  the  public,  two  tables 
which  centre  slides  accurately  in  both  directions.  One  was  in- 
vented by  C.  Mason  Kinne,  of  San  Francisco,  who  describes  it 
as  follows:  "As  will  be  seen  from  the  engravings,  Figs.  64  and 
65,  which  are  reduced  one-half,  the  slide  will  be  grasped  autom- 
atically, upon  removing  the  ringer  from  the  lever,  the  spiral 
spring  causing  the  clutches  to  instantly  clasp  the  slide,  and 
retain  it  in  a  central  position.  One  corner  of  either  end  of  the 
slide  projects  sufficiently  for  the  purpose  of  taking  hold  with 
one  Land,  while  the  other  is  pressing  the  lever,  and  can  be 
fixed  or  removed  without  pushing  along  a  circular  disc  to  its 
edge.  The  slots  are  made  to  allow  movement  enough,  so  that 


OF  THE  MICROSCOPE. 


213 


Fig.  64.— KHTXE'S  TURN-TABLE. 
(Upper  side.) 


Fig.  65.— KINNE'S  TURN-TABLE. 
(Underside.) 


the  clutches  can  grasp 
any  piece  of  glass  from 
l£  to  3£  inches  in  diag- 
onal length,  and  the 
table  is  made  of  brass 
about  a  quarter  of  an 
inch  thick,  -which  gives 
weight  sufficient  to  se- 
cure stability  of  move- 
ment. The  whole  rests 
on  a  small  spindle  4  or 
5  inches  long,  screwed 
into  the  centre  of  the 
brass  stud,  which  is  the 
fulcrum  of  the  lever, 
and  can  be  removed  at 
pleasure  to  pack  away. 
The  pointed  lower  end 
of  the  spindle  is  stepped 
into  a  counter-sunk 

metal  rest,  and  with  a  collar  placed  at  a  suitable  distance  above 
to  allow  of  free  movement  of  the  hand,  I  find  that  a  steady 
motion  can  be  obtained  with  the  thumb  and  finger,  of  any  re- 
quired velocity,  and  is  under  greater  control  than  with  any 
milled-head  device." 

Mr.  Kinne  suggests  a  very  simple  method  of  constructing  a 
home-made  table  on  this  plan:  "  The  spindle  can  be  fitted  into 
any  appliance,  primitive  or  expensive,  at  the  option  of  the 
worker,  and  I  find  that  an  old  cigar  box,  with  a  portion  of  one 
end  removed,  is  just  as  useful  as  anything  else,  though  if  made 
for  sale',  a  cheap  varnished  box  could  be  furnished,  and  in 
which  the  table  and  spindle  could  be  packed  when  desired.  If 
fitted  up  with  the  cast  iron  stand,  the  whole  might  present  a 
neater  appearance,  but  the  additional  expense  would  not  add  to 
its  utility." 

Slides  which  have  been  imperfectly  centered  on  other  tables, 
are  recentered  for  varnishing  by  the  use  of  two  rectangular  tri- 
angles and  a  little  wedge.  The  inventor  uses  the  corners  of  a 
broken  slide  and  a  piece  of  match. 


214 


SELECTION   AND    USE 


The  other  self-centering  turn-table  Wcis  invented  by  Mr.  C. 
F.  Cox,  of  New  York,  and  is  shown  in  Fig.  66,     The  slide  is 


Fig.  66. — COX  THEN-TABLE. 

grasped  by  two  angle-pieces,  which  are  simultaneously  moved 
to  and  from  the  centre  by  means  of  a  right  and  left  hand 
screw.  When  it  is  desired  to  re-varnish  slides  which  have 
not  been  accurately  centered  in  the  first  place,  a  pair  of  spring 
clips,  attached  to  a  stout  bar,  are  fastened  on.  This  can  be 
effected  in  an  instant.  The  arrangement  is  shown  in  Figo  67, 


Fig.    67.— COX  TURN-TABLE, 

There  is  also  a  very  ingenious  device  for  placing  a  row  of 
small  cells  along  the  middle  of  a  slide.  This  consists  of  two 
equal  right-angled  triangles,  the  square  corners  of  which  fit  into 


OF    THE    MICROSCOPE.  215 

the  clutches,  thus  allowing  the  long  sides  to  lie  parallel  to  each 
other,  and  at  equal  distances  from  the  centre.  A  slide  may 
thus  be  grasped  between  them,  and  pushed  along  longitudin- 
ally, as  may  be  desired. 

Those  who  once  see  a  turn-table,  will  find  no  difficulty  either 
in  understanding  the  method  of  using  it,  or  in  putting  this 
knowledge  into  practice.  The  slide,  being  held  on  the  table 
either  by  springs  or  clutches,  is  made  to  revolve  rapidly,  and  a 
brush,  charged  with  cement  or  varnish,  is  held  against  its  sur- 
face so  as  to  leave  a  ring.  There  is  a  slight  knack  about  making 
good  cells,  which  it  requires  a  little  practice  to  acquire.  The 
brush  must  be  held  in  the  direction  of  a  tangent  to  the  ring — 
that  is,  it  must  not  point  to  the  centre  of  the  circle,  but  must 
lie  so  that  the  ring,  as  it  revolves,  will  draw  the  cement  away 
from  the  brush.  Practice  alone  can  give  expeortness  in  doing 
this,  and  we  would  advise  the  beginner  to  work  steadily  for  a 
few  hours  at  making  cells  on  pieces  of  common  window  glass, 
strips  of  which  can  be  had  for  nothing  from  any  glazier.  The 
chief  points  to  be  attended  to  are  the  position  of  the  brush  and 
the  consistence  of  the  cement.  If  the  latter  be  too  fluid,  it 
spreads  and  does  not  form  a  well-defined  circle.  If  too  thick 
it  does  not  leave  the  brush  as  freely  as  is  necessary.  The 
method  of  preparing  the  cement  will  be  explained  under  the 
proper  heai 


Fig.  68. 

Where  a  turn-table  is  not  at  hand,  very  good  cells  may  be 
made  as  follows:  On  a  card  draw  the  outlines  of  a  slide  with  a 
series  of  circles  in  the  centre,  as  shown  in  Fig.  68;  lay  the 
slide  on  the  card  so  that  the  centre  of  the  circles  will  be  at  the 
centre  of  the  slide,  and  then  paint  a  circle  of  cement  on  the 


216  SELECTION  AND    USE 

slide  by  nand,  the  rings  beneath  serving  as  a  guictev  Very  good 
cells  may  be  thus  made,  but  the  process  is  of  course  more 
tedious  than  that  with  the  turn-table,  and  does  not  give  as  neat 
results. 

A  few  precautions  are  necessary  in  order  to  insure  the  per- 
manent adhesion  of  the  cells  to  the  glass.  In  addition  to  pro- 
viding cement  of  good  quality,  we  must  see  that  the  slide  is 
dry  and  recently  heated.  It  is  difficult,  with  most  cements,  to 
use  hot  slides,  as  the  cement  is  apt  to  flow;  but  the  slide  should 
have  been  recently  heated,  and  after  the  cement  has  partially 
hardened,  the  cells  should  be  baked  by  exposure  to  a  temper- 
ature as  high  as  they  will  stand.  This  is  easily  done  by  placing 
them  on  a  board  or  plate,  and  leaving  the  latter  for  a  short 
time  in  an  oven. 

Where  cells  of  greater  depth  are  required,  rings  of  various 
materials  are  cemented  to  the  slide.  For  objects  mounted  dry, 
rings  of  leather  or  cardboard  answer  well,  provided  they  are 
carefully  varnished  so  as  to  be  impervious  to  air  and  moisture. 
For  liquids,  rings  of  glass,  tin,  ebonite,  etc.,  are  used.  Eings 
of  rubber  and  gutta-percha  have  been  suggested,  but  they  do 
not  answer,  as  they  soon  become  rotten.  Full  directions  for 
making  and  using  deep  cells  may  be  found  in  the  works  of 
Quekett,  Carpenter,  Beale,  Frey,  etc. 

HOT-PLATE. — This  is  simply  a  stout  plate  of  brass  or  iron, 
which  is  supported  over  a  lamp  by  suitable  means.  The  com- 
mon plan  is  to  insert  four  stout  wires  to  serve  as  legs,  but  a 
better  mode  is  to  support  the  plate  on  the  ring  of  a  retort 
stand,  as  its  distance  from  the  lamp  can  thus  be  readily  ad- 
justed and  the  temperature  regulated.  The  hot-plate  serves  to 
distribute  the  heat,  and  thus  to  prevent  the  slides  from  becom- 
ing suddenly  and  unequally  heated.  Moreover,  by  means  of 
it  several  slides  can  be  heated  at  once,  and  thus  much  time 
may  be  saved.  It  should  be  tolerably  heavy.  The  one  we  use 
is  of  cast  iron,  six  inches  long  and  three  inches  wide.  The 
upper  surface  has  been  ground  so  as  to  be  tolerably  smooth, 
When  a  hot-plate  is  not  at  hand,  a  good  substitute  may  be 
found  in  a  smooth  brick,  or,  better  still,  a  plate  of  soapstcne. 
These  may  be  heated  in  the  fire  and  will  retain  their  heat  for  a 
long  time. 


OP  THE  MTCfSOSCOlTB.  217 

LAMP. — Any  lamp,  or  even  candle,  will  answer,  but  we  prefer 
a  spirit  lamp,  the  flame  being  free  from  smoke  and  easily  man- 
aged. At  night  the  kerosene  lamp  used  for  giving  light  will 
answer.  "Where  gas  is  used,  the  Bunsen  burner  is  a  great  con- 
venience. Whatever  lamp  or  burner  be  used,  it  should  be  sur- 
rounded with  a  chimney  or  shade,  so  as  to  prevent  the  flicker- 
ing of  the  flame  by  currents  of  air.  The  best  shade  is  a  tin 
cylinder,  with  rows  of  holes  at  top  and  bottom  for  the  admis- 
sion and  exit  of  air. 

RETORT  STAND. — a.  suitable  retort  stand  is  a  very  simple 
affair,  and  is  best  made  at  home.  Ours  consists  of  a  board  of 
hard  wood,  5  inches  by  4,  into  which  is  screwed  a  rod  fourteen 
inches  long,  and  a  quarter  of  an  inch  in  diameter.  The  rings 
have  no  screws,  but  are  simply  pieces  of  wire,  one  end  of  which 
is  twisted  round  the  rod,  while  the  other  is  formed  into  a  ring 
of  the  required  size.  Rings  formed  in  this  way  are  easily 
moved  on  the  upright  rod,  but  no  weight  placed  on  them  in 
the  usual  manner  can  cause  them  to  slip  down. 

CARDS  FOR  CENTERING  THE  OBJECTS. — Unless  the  objects 
are  placed  on  the  centres  of  the  slides,  the  latter  have  a  very 
awkward  look.  By  drawing  the  outlines  of  a  slide  on  a  card, 
and  marking  out  the  centre,  this  difficulty  is  easily  overcome. 
A  card  marked  off  in  this  way  is  shown  in  Fig.  69. 


Fig.  69. 

It  is  well  to  have  two  cards,  one  black  with  a  white  centre, 
and  the  other  white  with  a  black  centre,  as  some  objects,  when 
'mmersed  in  the  medium  in  which  they  are  to  be  mounted, 


218 


SELECTION   AND    USE 


show  best  against  a  dark  ground,  while  others  are  most  easily 
seen  against  a  light  one.  Those  who  use  the  self-centering 
turn-tables  may  readily  centre  their  slides  by  painting  on  them 
a  ring  of  some  water-color,  which  is  easily  washed  off.  The 
ring  is,  of  course,  laid  on  the  side  opposite  to  that  which 
receives  the  object. 

MOUNTING  NEEDLES. — These  are  similar  to  dissecting  needles, 
but  being  used  in  balsam,  varnish  and  similar  substances,  they 
cannot  be  used  for  dissection,  and  should  be  kept  by  them- 
selves. They  are  most  easily  cleaned  by  being  warmed  over  the 
lamp,  and  wiped  with  a  piece  of  soft  leather.  When  the  balsam 
is  burned  on  them,  as  recommended  by  some,  it  leaves  a  crust 
which  is  not  easily  removed. 

COVEB  FOKCEPS. — In  placing  the  cover  on  the  object,  the 
ordinary  forceps  are  very  inconvenient.  We 
have  long  used  a  pair  of  forceps  bent  as  in 
Fig.  70,  and  with  the  points  carefully 
adjusted.  The  mode  of  using  the  instrument 
will  be  obvious  from  the  engraving.  A  very 
ingenious  device  intended  to  answer  the  same 
purpose  has  been  invented  by  Dr.  Fletcher. 
These  forceps  are  self-closing,  so  that  the 
thin  glass  cover  is  held  without  any  effort. 
After  the  cover  is  in  position  on  the  slide, 
by  pressing  on  the  blades  they  open  and  allow  it  to  slip  out. 
If  the  cover  should 
stick  to  the  forceps 
in  the  slightest  de- 
gree, it  may  be  pre- 
vented from  moving 
when  the  forceps 

are  removed  by  in-  Fig.  71. 

serting  a    common 

pin  in  the  slit  seen  in  Fig.  71.  When  using  the  forceps 
shown  in  Fig.  70,  the  same  end  may  be  attained  by  means  of  a 
wire  fork  (a  hair-pin  is  as  good  as  anything),  which  may  be 
made  to  straddle  the  nose  of  the  instrument. 


Fig.  70. 


OF  THE  MICBOSCOPE. 


219 


SUDE  HOLDER. — The  hot  slides  cannot  be  comfortably  held 
in  the  fingers,  and  therefore  a  pair  of  wooden  forceps  become 
a  necessity.  Those  usually  sold  are  made  by  screwing  together 
two  thin  slips  of  wood  with  a  piece  of  brass  or  lead  inserted  be- 
tween them  at  one  end.  To  admit  the  slide,  the  slips  are  forced 
apart  by  pressing  on  pins  arranged  as  in  the  stage  forceps* 
"When  placed  on  a  table  the 
metal  counter-balances  the  slide, 
and  keeps  it  from  touching  the 
surface  on  which  it  is  laid — a 
very  important  point.  The  Eng- 
lish forceps,  being  all  wood,  fre- 
quently tip  with  a  heavy  slide. 

A  common  spring  clothes-pin 
is  frequently  used,  but  when  we  <£• 
come  to  lay  the  slide  down,  the  ^ 
clothes-pin  holds  it  in  an  awk- 
ward manner.  The  end  of  the 
hot  slide  is  sure  to  lie  on  the  table, 
and  if  fluid  balsam  or  other  me- 
dium should  be  present,  the  fact  § 
that  the  slide  is  not  level  produces  g 
bad  results.  By  cutting  off  about 
half  an  inch  from  one  of  the 
limbs  of  the  forceps  part  of  the  g 
pin,  however,  this  difficulty  is 
avoided.  The  slide  may  then  be 
grasped  in  such  a  way  that  when 
the  clothes-pin  is  placed  on  the 
table,  the  glass  will  be  held  in  a 
perfectly  level  position.  A  glance 
at  Fig.  72  will  show  what  we 
mean.  A  great  advantage  of  this 
form  of  holder  is  that  it  costs  but  a  trifle,  so  that  the  micro- 
scopist  can  supply  himself  with  an  abundance  of  them,  and 
thus  several  slides  may  be  cooling,  while  work  on  others  is 
going  on.  When  very  heavy  slides  are  used,  it  may  become 
necessary  to  screw  a  plate  of  sheet  lead  to  the  under  side  of  the 
clothes-pin,  so  as  to  prevent  tipping 


220 


SELECTION  AND    USE 


WATER  BATH. — A  water  bath  is  indispensable  in  those  cases 
where  a  certain  very  moderate  degree  of  heat  is  not  to  be  ex- 
ceeded. Few  persons  fully  appreciate  the  difficulty  of  regu- 
lating or  even  estimating  the  temperature  of  an  object  held  over 
a  naked  flame,  and  mischief  is  often  done  before  the  operator 
is  aware  of  it.  A  serviceable  water  bath  is  easily  extemporized 
out  of  an  old  fruit  can  and  a  small  beaker  glass.  This  serves 
for  exposing  material  and  preparations  to  a  temperature  lower 
than  that  of  boiling  water.  Where  slides  are  to  be  so  heated, 
the  simplest  contrivance  is  a  flat  tin  box,  with  all  the  joints 
(cover  and  all,  of  course,)  tightly  soldered.  A  small  tube, 
closed  with  a  cork,  serves  to  admit  the  water. 

SPEING  CLIPS. — One  of  the  first  of  the  needs  which  impress 
themselves  upon  the  mind  of  the  beginner,  is  the  necessity  foi 
something  to  retain  the  thin  cover  in  its  place,  until  the  ce- 
ment, which  is  intended  to  hold  it  permanently,  dries.  An  end- 
less variety  of  spring  clips  have  been  invented  for  this  purpose, 
but  we  have  never  seen  anything  that  we  liked  better  than  the 
simple  article  shown  in  Fig.  73,  and  which  we  have  used  for 


Fig.  73. 

over  fifteen  years.  It  consists  simply  of  a  piece  of  brass  wire 
bent  as  in  the  engraving.  The  slide  being  held  in  the  left 
hand,  the  clip,  held  by  the  upper  wire,  is  brought  so  that  the 
projecting  part  of  the  ring  is  placed  under  the  edge  of  the 
slide.  The  upper  part  is  then  lifted  up  so  as  to  open  the  clip, 
which  is  then  slid  on  to  the  slide  until  the  vertical  point  is  in 
the  right  position.  When  a  broader  surface  than  the  point  of 
the  wire  is  needed,  a  piece  of  cork  may  be  stuck  on  it,  and  if 


OF  THE  MICROSCOPE.  221 

there  should  be  need  for  greater  pressure  than  that  which  the 
spring  of  the  wire  affords,  this  can  be  obtained  by  sliding  a 
small  brass  ring  on  to  the  clip. 

Various  other  forms  of  spring  clip  have  been  invented,  but 
none  that  we  consider  more  simple,  or  that  we  like  better  than 
the  above,  which  has  this  great  merit,  that  any  one  can  make  it 
for  himself  out  of  materials  that  may  be  obtained  at  any  hard- 
ware store.  It  must  be  borne  in  mind,  however,  that  all  clips 
constructed  upon  this  plan  are  apt  to  cause  a  slight  displace- 
ment of  the  object,  from  the  fact  that  the  movement  of  the 
point  is  not  quite  perpendicular.  "With  delicate  objects  this  is 
a  matter  of  importance.  The  only  remedy  is  to  use  the  end 
pressure  of  a  rod  moving  in  fixed  guides. 


CEMENTS  AND  VARNISHES. 

A  supply  of  carefully  selected  cements  and  varnishes  is  indis- 
pensable to  the  microscopist,  and  it  is  also  well  that  he  should 
understand  the  nature  and  properties  of  the  materials  used, 
otherwise  he  will  be  liable  to  make  gross  blunders.  Thus,  of 
the  different  articles  in  use,  some  are  easily  mixed  with  each 
other,  while  others  separate  as  soon  as  left  to  themselves;  some 
dry  in  one  way  and  some  in  another.  It  would  require  a  vol- 
ume to  detail  the  properties  of  the  different  substances  which 
enter  into  the  composition  of  the  cements  used  by  the  micro- 
scopist. We  have  space  for  only  the  following  hints,  which, 
however,  we  hope  will  prove  useful. 

Cements  become  hard  in  three  different  ways — cooling,  evap- 
oration and  oxidation.  Shellac,  sealing  wax,  electrical  cement, 
etc.,  when  melted  by  heat,  furnish  examples  of  the  first  pro- 
cess. Shellac  and  sealing-wax  dissolved  in  alcohol,  and  asphalt 
and  damar  dissolved  in  turpentine,  dry  by  the  second  process — 
the  solvents  evaporating  and  leaving  behind  the  material  which 
they  had  dissolved.  Drying  oil  in  all  its  forms,  such  as  gold 
size,  paint,  etc. ,  becomes  hard  by  oxidation — not,  as  is  gener- 
ally supposed,  by  evaporation. 

In  the  case  of  varnishes  which  dry  by  the  evaporation  of 
some  of  their  constituents,  it  is  obvious  that  if  a  fresh  layer 


222  SELECTION   AND    USE 

be  laid  orer  an  old  one,  the  old  layer  will  be  softened,  and 
if  there  should  be  any  tendency  to  a  vacuum  in  the  cell, 
>_>j  softened  cement  will  be  unable  to  resist  the  outside 
^essure,  and  will  creep  in  and  spoil  the  object.  So,  too, 
with  varnishes  or  cements  formed  chiefly  of  drying  oil  or  gold 
size."  If  the  different  coats  be  laid  on  too  thickly  or  too 
rapidly,  the  part  that  is  beneath  cannot  easily  harden,  but  will 
remain  for  a  very  long  time  in  a  semi-liquid  condition.  We 
have  just  removed  some  brass  rings  from  slides  to  which  they 
were  attached  four  months  ago  by  means  of  gold  size,  and 
although  the  outer  surface  of  the  cement  was  hard  and  dry,  the 
interior  was  quite  liquid,  freely  soiling  the  fingers. 

GOLD  SIZE. — The  most  extraordinary  recipes  have  been  given 
for  the  preparation  of  this  cement,  which  is  in  reality  nothing 
but  good  linseed  oil  rendered  very  drying  by  the  usual 
methods.  Gilders  frequently  make  it  into  a  semi-paint  by 
adding  coloring  matter,  thus  forming  a  ground  of  a  shade 
similar  to  the  gold  they  use,  and  this  seems  to  have  misled  some 
of  our  best  writers.  There  is  no  ochre,  litharge,  or  anything 
of  the  kind  present  in  good  gold  size.  It  does  not  pay  to  pre- 
pare gold  size  in  small  quantities,  and  it  may  be  obtained  from 
any  color  dealer.  The  older  it  is  the  better,  and  it  is  well, 
therefore,  to  lay  in  a  good  stock,  which  must  be  kept  carefully 
corked.  The  working  supply  should  be  kept  in  a  small  bottle. 
This  is  the  favorite  cell  making  material  employed  by  Dr.  Car- 
penter, and  it  is  certainly  the  most  reliable  cement  we  have. 
'  It  adheres  firmly  to  glass,  and  if  laid  on  in  very  thin  successive 
layers,  tolerably  deep  and  very  durable  cells  may  be  built 
up,  but  the  process  requires  considerable  time,  otherwise  the 
under  layers  will  remain  soft.  It  has  this  great  advantage  over 
asphalt,  damar,  and  other  cements  composed  of  solid  materials 
dissolved  in  some  menstruum,  that  fresh  coats  have  but  very 
slight  action  on  the  old  layers  on  which  they  may  be  laid.  It 
mixes  with  turpentine,  and  consequently  with  most  materials 
soluble  in  turpentine,  but  when  once  dry  and  hard,  turpentine, 
alcohol,  ether,  etc.,  have  little  or  no  action  on  it.  It  does  not 
mix  with  alcohol,  and  therefore  cannot  be  mixed  with  the 
solution  of  shellac  in  alcohol  in  any  of  its  forms. 


OF  THE  MICROSCOPE.  223 

BLACK  JAPAN. — When  this  can  be  procured  of  good  quality, 
it  makes  a  very  excellent  cell  It  adheres  very  firmly  to  the 
glass  provided  the  latter  be  exposed  to  a  moderate  heat  after 
the  cement  has  become  dry.  Black  Japan  dries  tip  and  thick- 
ens when  kept,  but  may  be  thinned  with  turpentine. 

BRUNSWICK  BLACK. — This  is  simply  a  solution  of  asphalttim 
in  turpentine.  Occasionally  it  is  rendered  very  black  by  the 
addition  of  a  little  lampblack.  When  good,  it  makes  a  very 
excellent  cement.  Its  quality  depends  chiefly  upon  the  char- 
acter of  the  asphaltum  that  is  used  in  its  preparation.  Now 
there  are  several  varieties  of  asphaltum  in  market,  the  most 
common  kind  at  the  present  day  being  that  obtained  from  coal 
tar.  This  seems  to  be  entirely  unfit  for  the  purpose.  The 
proper  kind  is  that  which  is  found  native  in  several  parts  of  the 
world.  The  two  kinds  are  easily  distinguished  by  their  odors.. 

SHELLAC. — This  well  known  substance,  when  dissolved  in 
alcohol,  forms  a  varnish  or  cement  of  great  value  to  the  micro- 
scopist,  and  is  the  proper  one  to  be  used  when  glycerine  is  em- 
ployed. Much  of  the  shellac  in  market  is  artificially  made 
from  resin  and  wax,  and  makes  a  poor  varnish.  Real  shellac 
must  be  employed  if  failure  would  be  avoided. 

BELL'S  CEMENT. — Carpenter  states  that  this  cement  is  merely 
shellac  dissolved  in  alcohol.  With  us  it  has  presented  no  ad- 
vantage over  other  cements. 

SEALING  WAX  VARNISH. — This  is  prepared  by  dissolving  the 
best  sealing  wax  in  alcohol.  It  unfortunately  happens  that  all 
the  fancy  colored  sealing  wax  in  market  is  of  inferior  quality. 
Very  excellent  red  wax  may  be  obtained,  but  we  have  never 
been  able  to  obtain  good  blue,  black,  green  or  other  colored 
wax.  We  therefor©  make  varnish  of  these  colors  by  dissolv- 
ing in  alcohol  the  materials  used  for  making  the  best  red  wax, 
substituting  some  other  color,  however,  for  the  vermillion. 

COLORED  SHELLAC. — Bleached  shellac,  dissolved  in  alcohol, 
and  colored  witi  aniline  blue,  red,  etc.,  makes  a  very  fine  trans- 
parent varnish. 


224  SELECTION  AND     USE 

DAMAR  CEMENT. — This  is  a  mixture  of  equal  parts  of  damar 
7arnish  and  gold  size,  mixed  together.  It  should  stand  for 
some  time  before  being  used.  It  is  said  to  be  very  excellent. 
It  is  very  tough,  and  serves  well  as  an  outer  coating  over  such 
brittle  cements  as  shellac  and  sealing  wax  varnish. 

MARINE  GLUE. — This  is  undoubtedly  the  strongest  cement 
in  use  for  joining  pieces  of  glass  or  glass  and  metal  together. 
Skilful  microscopists  make  great  use  of  it;  beginners  do  not 
find  it  so  easy  to  manage  as  some  others.  In  using  it,  the 
simplest  method  is  to  cut  it  in  small  pieces,  lay  it  on  one  of  the 
surfaces  that  are  to  be  joined,  melt  it  by  heat,  and  apply  the 
other  surface,  making  sure  of  perfect  contact  by  rubbing  the 
two  pieces  upon  each  other,  if  they  will  allow  of  it.  Marine 
glue  may  be  obtained  from  most  dealers  in  microscopes. 

The  cement  known  as  liquid  glue,  is  simply  a  solution  of 
shellac  in  alcohol. 

For  attaching  labels,  paper  covers,  etc.,  to  the  slides,  nothing 
is  better  than  good  dextrine.  After  having  mixed  the  dextrine 
with  water  to  the  proper  consistence,  add  six  drops  of  glycer- 
ine to  the  fluid  ounce  of  dextrine.  This  will  prevent  the  labels 
or  covers  from  cracking  off. 

Having  provided  himself  with  the  necessary  tools  and  mater- 
ials, the  next  step  is  to  learn  how  to  use  and  apply  them,  and  this 
will  probably  be  most  easily  taught  by  describing  a  few  char- 
acteristic examples.  And  first  of  all,  selecting  the  most  easily 
mounted  of  all  objects,  we  commence  with  the  scales  on  the 
butterfly's  wings.  Having  prepared  a  cell  of  proper  size,  and 
allowed  it  to  dry,  the  first  step  is  to  select  a  cover-  to  suit  it, 
and  give  a  final  cleaning  to  both  slide  and  cover.  When  every 
particle  of  dust  has  been  removed,  breathe  gently  on  the  slide, 
and  press  the  wing  lightly  against  it,  and  within  the  cell.  A 
large  number  of  scales  will  at  once  adhere  to  the  slide,  and 
the  next  step  is  to  attach  the  cover.  Place  the  slide  on  the  hot- 
plate, (which  must  not  be  too  hot,  however,)  and  when  it  is 
thoroughly  dry,  and  the  cement  somewhat  soft,  lay  the  cover 
on  by  means  of  the  cover  forceps.  Press  it  into  contact  with 
the  cement,  and  the  operation  is  completed.  It  is  not  difficult 
to  see  when  the  cover  and  the  cement  are  in  perfect  contact, 


OP  THE  MICBOSCOPE.  225 

and  great  care  must  be  taken  to  close  the  cell  all  round  in  this 
way.  It  is  true,  this  point  is  not  of  so  much  consequence  witli 
the  particular  object  under  consideration,  but  with  some  ob- 
jects it  would  be  quite  important.  The  scales  are  now  mounted 
dry,  and  may  be  kept  for  any  length  of  time;  no  dust  can  soil 
them,  and  they  are  not  liable  to  be  injured  by  contact  with 
other  bodies.  It  only  remains  to  label  and  "  finish  "  the  slide 
as  hereafter  directed. 

Next  to  the  above  in  simplicity  is  the  mounting  of  such  ob- 
jects  as  the  wings  of  insects  in  balsam.  Suppose  we  wish  to 
mount  one  of  the  smaller  wings  of  a  bee  or  wasp,  so  as  to  show 
the  curious  hooks  with  which  it  is  armed:  Place  the  warm  slide 
on  the  centering  card,  drop  a  little  balsam  on  the  centre,  and 
again  warm  the  slide,  so  that  any  air  that  may  be  present  may 
collect  in  fiue  bubbles  which  can  be  removed  by  means  of  a 
cold  mounting  needle.  When  the  air-bubbles  have  been  re- 
moved, seize  the  wing  (previously  well  cleaned  with  a  camel 
hairbrush)  with  a  pair  of  fine  forceps,  and  lower  the  tip  of  it 
i  nto  the  warm  balsam.  Then  slowly  lower  the  wing  until  it  is  en- 
tirely immersed.  Drop,  very  little  more  balsam  on  it,  warm 
the  slide  again  (slightly  this  time),  and  remove  air  bubbles  if 
there  should  be  any.  Then  take  a  clean  cover  in  the  cover  for- 
ceps, make  it  quite  warm,  and  place  it  over  the  object  by  allow- 
iugit  to  first  touch  one  edge  of  the  balsam,  and  then  to  grad- 
ually fall  down  so  as  to  exclude  <ill  air  bubbles.  In  the  case 
of  the  bee's  wing  it  does  not  answer  to  apply  much  pressure  as 
this  would  tend  to  distort  the  hooks.  Press  the  cover  into 
place  as  much  as  it  will  bear  and  no  more,  lay  the  slide  in  a 
warm  place  for  some  time  until  the  balsam  hardens,  and  then 
clean  and  finish  the  slide. 

In  mounting  objects  in  balsam  aud  fluids,  the  great  difficulty 
to  be  encountered  is  the  presence  of  air  bubbles.  Careful  and 
j  udicious  management,  however,  readily  enables  us  to  avoid 
them.  In  the  first  place  see  that  they  are  entirely  removed 
from  the  balsam  on  the  slide.  This  is  much  more  easily  done 
before  immersing  the  object  in  the  balsam  then  afterwards. 
Next  see  that  the  air  is  expelled  from  the  object.  In  the  case 
of  the  wing,  this  is  effected  by  slowly  immersing  the  object  in 
hhe  balsam.  Lastlv  see  thut  no  air  enters  with  the  cover.  To 


226  SELECTION  AND    tJSE 

do  this  see  that  the  cover  is  hot,  and  that  it  is  lowered  on  the 
balsuni  slowly,  and  from  one  side.  If  in  any  case  there  should 
be  a  vacant  space  under  the  cover  as  at  a  Fig.  74,  and  it  should 
be  desired  to  fill  it,  do  not  apply  the  fresh  balsam  directly  at  a. 
To  do  so  would  certainly  be  to  inclose  a  large  air  bubble.  Drop 
the  balsam  at  Z>,  warm  the  slide,  and  the  balsam  will  creep  in  by 
capillary  attraction,  and  expel  the  air. 


Fig.  74. 

Let  us  now  suppose  that  we  have  some  small  insect  which 
we  have  prepared  by  soaking  in  potash,  and  which  we  desire 
to  mount  in  balsam.  Such  a  preparation  if  immersed  directly 
in  balsam,  would  be  spoilt,  since  the  balsam  and  watery  solu- 
tion would  not  mix.  Therefore,  proceed  as  follows:  Wash  the 
insect  in  pure  water,  and  drain  off  the  water;  wash  with  strong 
alcohol,  drain  off  the  alcohol,  and  soak  for  twenty-four  hours 
in  the  strongest  alcohol  you  can  get.  Pour  off  the  alcohol  and 
soak  for  twenty-four  hours  in  turpentine.  The  object  may 
now  be  immersed  in  balsam  without  difficulty. 

Air  pumps  and  similar  contrivances  are  generally  recom- 
mended as  the  best  means  for  removing  air  bubbles,  but  we 
never  use  them.  If  the  object  be  dry,  we  soak  it  in  alcohol 
until  all  the  air  has  been  expelled,  then  transfer  to  turpentine, 
and  finally  to  balsam.  This  requires  time,  it  is  true  but  it  does  not 
occupy  the  time  of  the  microscopist.  The  soaking  process 
goes  on  without  any  attention  from  him,  and  while  it  involves 
far  less  labor,  with  us  it  has  always  given  far  better  results, 
though  we  have  used  very  fine  air  pumps,  and  followed  the 
best  published  directions.  Take  the  case  of  a  dry  shaving  of 
wood,  many  of  which  are  well  worth  mounting.  It  would  be 


OF  THE  MICROSCOPE.  227 

« 

very  laborious  to  get  the  air  out  of  this  by  means  of  the  air 
pump,  while  by  soaking  successively  in  water,  alcohol,  and  tur- 
pentine, it  can  be  mounted  with  great  ease  without  a  bubble. 

Let  us  now  take  the  c  se  of  an  object  mounted  in  fluid  in  a 
cell.  Suppose  it  is  the  so-called  tongue  of  a  fly,  which  of  course 
has  been  soaked  for  some  time  in  the  liquid  in  which  it  is  to  be 
finally  mounted,  viz.,  dilute  glycerine.  We  make  a  cell  of  suita- 
ble thickness,  which  in  this  case  may  be  made  with  shellac 
dissolved  in  alcohol.  Several  coats  will  be  required,  and  as 
shellac  alone  does  not  adhere  well  to  glass,  we  prefer  to  lay  on 
first  a  coat  of  gold  size  or  Japan,  and  when  this  is  thoroughly 
dry,  to  lay  the  shellac  on  it.  No  difficulty  will  be  found  in 
making  a  cell  of  sufficient  depth.  The  cell  is  now  to  be  filled 
with  the  liquid,  the  object  placed  in  it,  and  the  whole  carefully 
examined  for  air  bubbles,  which  must  be  removed  if  they  exist. 
The  cover  is  now  applied,  all  superfluous  fluid  removed  by 
means  of  a  camel  hair  pencil,  which  has  been  moistened  and 
then  squeezed  dry,  and  finally  the  edge  of  the  cover  is  to  be 
coated  with  a  thin  layer  of  cement.  After  a  day  or  so  another 
layer  of  cement  should  be  laid  on,  and  this  process  repeated 
until  at  least  three  layers  have  been  applied. 

We  give  no  directions  for  the  construction  and  use  of  very 
deep  cells  as  this  is  work  that  will  hardly  be  attempted  by  be- 
ginners. 

When  opaque  objects  are  to  be  mounted  either  in  balsam,  or 
in  fluid,  the  process  required  is  the  same  as  that  employed  for 
transparent  objects.  Very  many  opaque  objects  are,  however, 
mounted  dry,  and  in  this  case  all  that  is  needed  is  to  attach 
them  to  a  slide,  and  see  that  they  are  properly  protected. 
When  thin  they  may  be  readily  mounted  in  cement  cells,  and 
this  is  altogether  the  neatest  and  most  secure  plan  where  it  can 
be  used.  Thicker  objects  require  deeper  cells,  which  may  be 
made  of  card,  ebonite  or  electrical  cement.  (3  parts  resin,  and 
1  of  wax,  colored  with  ochre  or  any  similar  matter).  Cells  of 
card  are  made  by  first  punching  out  a  disc  like  a  gun  wad, 
and  then  punching  a  hole  in  this  so  as  to  leave  a  ring.  The 
ring  is  to  be  cemented  to  the  glass  slide  and  carefully  var- 
nished. 

Wooden  slides  with  a  cell  bored  in  the  centre,  are  recom- 


SELECTION  AND    USB 

mended  very  highly,  and  seem  to  answer  a  very  good  purpose. 
The  cells  are  not  bored  quite  through  the  wooden  slip,  and  as 
they  are  blackened  on  the  inside,  any  small  object  that  may  be 
cemented  to  the  bottom  of  them  shows  very  well.  For  seeds, 
small  shells,  and  similar  objects,  they  answer  admirably.  In 
most  cases  it  will  be  found  unnecessary  to  cover  the  cells  with 
thin  glass.  Several  slides  may  be  packed  together  face  to  face, 
and  if  held  in  firm  contact  by  means  of  a  rubber  ring,  dust  will 
be  entirely  excluded.  Or  they  may  be  arranged  in  the  drawers 
of  an  ordinary  cabinet,  face  down,  the  labels  being  placed  on 
the  backs.  This  will  effectually  exclude  the  dust. 

Some  years  ago  we  mounted  a  large  number  of  specimens  of 
minerals  on  leather  discs,  which  were  cemented  to  glass  slides. 
These  leather  discs  were  three-quarters  of  an  inch  in  diameter, 
and  we  had  a  lot  of  pill-box  covers  which  exactly  fitted  them. 
These  covers,  when  slipped  on  to  the  discs,  protected  the  ob- 
jects perfectly,  and  the  whole  formed  a  very  cheap,  convenient 
and  excellent  mode  of  mounting. 

A  very  ingenious  cell  for  opaque  objects,  the  invention  of 
Prof  Pierce,  of  Provi- 
dence, R.  I.,  is  shown  in 
Tig.  75.  It  consists  of  a 
metallic  cell,  having  a 
broad  flange  like  the 
rim  of  a  hat,  which  is 
cemented  to  an  ordinary  Fig.  75. 

glass  slide,  as  shown  in 

section  in  the  lower  figure.  To  this  cell  is  fitted  a  metal  cap, 
which  covers  and  protects  the  object.  The  object  may  be  placed 
directly  on  the  glass,  or  raised  by  means  of  a  disc  of  any  re- 
quired thickness,  so  as  to  be  more  easily  illuminated.  The 
slide,  with  cell  uncovered  and  containing  an  object,  is  shown 
in  the  upper  figure.  Uncovered  objects  may  in  this  way  be 
very  perfectly  protected  from  dust  and  mechanical  violence. 

Some  persons  object  to  any  slide  that  is  mounted  without  a 
glass  cover.  It  must  -be  acknowledged,  however,  that  while 
glass  covers  add  to  the  appearance  of  the  object  and  serve  to 
protect  it,  they  interfere  somewhat  with  its  examination,  as  it 
cannot  be  so  brilliantly  illuminated,  and  the  rays  in  their  passage 


OP  THE   MICROSCOPE.  229 

to  the  objective  are  interfered  with  to  a  considerable  extent. 
The  objections  to  mounting  opaque  objects  in  cells  with  mov- 
able covers  are  that  even  during  the  short  period  in  which  they 
are  exposed  for  examination  they  are  liable  to  be  contaminated 
with  dust,  the  cover  is  liable  to  be  lost,  and  the  object  when 
exposed  is  liable  to  mechanical  injury.  Therefore,  while  a  few 
brilliant  and  striking  objects,  such  as  minerals,  seeds,  etc.,  may 
well  be  mounted  in  open  cells,  all  delicate  and  valuable  objects 
should  be  permanently  covered. 

Of  late  years  the  most  popular  cell  for  opaque  objects  is  un- 
doubtedly that  devised  by  Prof.  Hamilton  L.  Smith,  and  known 
as  the  wax  cell.  Various  methods  of  making  it  are  in  use,  the 
following  being  that  originally  published  by  the  inventor: 
Take  a  circular  disc  of  thin  sheet  wax,  which  is  easily  cut  with 
a  common  gun  punch  from  the  sheet  wax  ordinarily  used  for 
making  flowers,  and  attach  it  by  means  of  heat  to  the  cen- 
tre of  a  glass  slide.  A  brass  curtain  ring,  of  which  the  inte- 
rior is  the  same  size  as  the  disc,  is  then  slightly  warmed  and 
laid  on  the  wax;  to  which  it,  of  course,  adheres.  The  object  is 
fixed  to  the  wax  by  slightly  moistening  the  surface  of  the  latter 
by  a  minute  drop  of  turpentine.  When  dry,  a  cover,  which 
exactly  fits  into  the  bevel  of  the  ring  is  attached  thereto  with  a 
little  cement,  and  the  whole  may  then  be  finished  off  on  the 
turn-table. 

The  appearance  of  objects  mounted  in  this  way  is  very  ele- 
gant, and  consequently  it  has  become  a  general  favorite.  It 
has,  however,  recently  been  condemned  in  most  unsparing 
terms  by  the  inventor,  who  has  found  that  the  under  surface 
of  the  cover  becomes  in  time  coated  with  a  deposit  which 
causes  a  glare  that  entirely  prevents  clear  vision.  As  soon  as 
this  difficulty  was  announced,  an  animated  correspondence 
took  place  in  the  scientific  journals,  and  it  was  found  that 
while  some  microscopists  had  experienced  this  difficulty,  it 
had  never  occurred  to  others.  The  final  conclusion  seems  to 
be  that  the  difficulty  arises  from  the  kind  of  wax  used,  and  the 
method  of  attaching  the  object  to  it.  The  wax  should  have 
been  thoroughly  melted  at  a  temperature  as  high  as  it  will 
bear,  so  as  to  drive  off  all  volatile  matter,  and  instead  of  using 
discs  punched  from  sheets,  the  wax  should  be  applied  in  a 


230 


SELECTION    AND    USE 


melted  condition,  by  means  of  a  brush,  to  the  warm  glass  slide. 
The  object  should  not  be  stuck  on  with  turpentine  or  similar 
cement,  but  should  be  attached  to  the  wax  after  a  small  spot 
on  the  latter  has  been  softened  by  bringing  near  it  a  hot  wire. 
Objects  mounted  in  this  way,  in  cells  so  prepared,  seem  to  re- 
main in  good  condition  for  years. 

A  very  ingenious  cell  has  been  devised  by  Mr.  D.  B.  Scott. 
This  cell  is  punched  out  of  thin  sheet  metal,  as  shown  in  plan 
and  section  in  figures  76  and  77.  The  cell  is  formed  by  the 


Fig.  76. 


Fig.  77. 

METAL  SLIDE  AND   CELL  FOR  OPAQUE   OBJECTS. 

central  depression,  and  there  is  a  turned-down  edge  all  round 
the  slide  which  gives  it  strength,  and  causes  it  to  lie  steadily 
on  any  flat  surface.  The  cell  has  a  ledge,  or  rebate,  as  seen  in 
figure  77,  for  the  purpose  of  supporting  the  thin  glass  cover. 
When  made  of  tin  the  whole  slide  is  japanned;  those  made  of 
brass  are  lacquered,  and  the  interior  of  the  cell  is  covered  with 
black  asphalt,  or  some  similar  dark  varnish.  The  objects  are 
attached  to  the  surface  of  the  varnish  by  means  of  gum  water, 
to  which  a  very  little  glycerine  has  been  added,  and  the  thin 
glass  cover  may  be  cemented  down  and  varnished  on  the  turn- 
table in  the  usual  manner. 


Fig.  78. — CELL  FOR  OPAQUE  OBJECTS. 

The  alleged  failure  of  the  wax  cell  gave  rise  to  a  great  many 
devices,  one  of  which,  proposed  by  Mr.  Atwood,  consists  of  a 
vulcanite  or  hard  rubber  cell,  of  which  a  sectional  view  is 
given  in  figure  78,  the  dotted  lines  showing  the  thin  glass 


OF  THE  MICROSCOPE.  231 

cover.  The  base  is  solid,  thus  giving  a  black  back-ground  of 
rubber;  around  the  top  is  a  ledge  fitted  to  receive  a  one-half 
inch  cover  gloss;  this  being  secured  by  a  trifle  of  shellac  or 
any  similar  cement,  completes  the  mounting.  The  cell  may 
be  attached  to  a  glass  slip  by  any  cement,  before  or  after 
preparation.  For  exchanges  it  offers  superior  advantages,  in- 
asmuch as  the  cell,  with  objects  enclosed,  may  be  sent  through 
the  mails  independent  of  the  glass  slips,  the  recipient  attach- 
ing them. 

Cells  similar  in  construction  to  the  hard  rubber  cell  may  be 
moulded  out  of  melted  shellac  by  any  one  who  is  possessed  of 
a  proper  die  or  mould.  The  die  is  easily  turned  out  of  a  piece 
of  brass,  and  with  two  or  three  moulds  of  different  sizes,  and  a 
little  shellac  colored  black,  the  microscopist  may  easily  and 
cheaply  provide  himself  with  a  supply  of  cells  suitable  for 
almost  any  object.  The  idea  is  due  to  Dr.  Dayton,  of  Cleve- 
land, and  the  details  of  the  process  may  be  found  in  the  Ameri- 
can Journal  of  Microscopy  for  June,  1881. 

A  cell  which  we  have  found  very  durable,  easily  and  quickly 
made,  and  very  neat,  is  constructed  as  follows:  Having  pro- 
cured some  good  gold  size  and  pure  litharge,"  grind  the  latter 
to  a  very  fine  powder.  Mix  the  litharge  and  gold  size  to  the 
thickness  of  cream,  and  color  either  black  or  dark  olive  by 
adding  lamp-black.  With  this  cement  it  is  easy  to  make  as 
many  cells  as  may  be  wanted,  by  laying  on  a  ring  with  a  brush 
while  the  glass  slide  revolves  on  the  turn-table.  As  soon  as 
the  rings  or  cells  are  made,  dust  finely-powdered  litharge  over 
them  until  they  are  covered  a  sixteenth  of  an  inch  deep;  allow 
them  to  stand  a  few  minutes,  and  then  shake  off  all  the  loose 
litharge  by  means  of  a  few  smart  taps.  The  surface  of  the  cell 
will  now  be  quite  rough.  Allow  it  to  stand  a  few  hours,  and 
then  press  it  against  a  p  ate  of  glass.  If  this  be  done  carefully, 
a  smooth,  level,  solid  ring  will  be  left  on  the  slide.  If  the 
edges  should  not  be  as  smooth  as  they  ought  to  be,  it  is  easy 
to  trim  them  off  on  the  turn-table  by  means  of  a  small  chisel. 
Any  turn-table  with  stout  spring  clips  will  held  a  slide  with 
sufficient  firmness  to  allow  of  such  soft  material  being  turned 
quite  true  and  smooth.  Of  course  the  tables  which  grasp  the 
slides  by  the  corner  are  best  for  this  purpose.  Such  cells,  after 


232 


SELECTION   AND   USE 


a  few  weeks,  become  very  hard,  and  may  be  finished  so  as  to 
be  very  neat.  When  covered  with  a  few  coats  of  shellac  var- 
nish or  pure  gold  size,  and  completely  dried,  they  hold  liquids 
very  well.  They  adhere  so  firmly  to  the  slide  that  on  several 
occasions  when  the  slide  has  been  broken  by  a  fall,  the  cell  has 
not  parted  from  the  glass.  The  only  objection  which  we  find 
to  them  is  the  length  of  time  which  it  takes  them  to  harden. 

Tin  foil,  which  may  be  had  of  various  thicknesses  from  that 
of  thin -paper  to  a  sixteenth  of  an  inch,  makes  a  capital  mate- 
rial for  cells.  It  is  difficult  to  get  the  inner  and  outer  circles 
which  form  the  ring,  concentric,  except  by  the  use  of  special 
tools.  Prof.  Chester  avoids  this  difficulty  by  placing  a  large 
number  of  rings  on  a  rod  or  mandril  which  just  fits  the  open- 
ing, and  after  screwing  the  rings  tightly  endwise,  he  turns  off 
the  outside  so  as  to  leave  it  perfectly  true  and  even.  Mr.  A.  Y. 
Moore  cements  the  sheet  of  tin  foil  to  the  slide  by  means  of 
shellac,  and  cuts  out  the  ring  on  the  turn-table  by  means  of  a 
sharp  knife  or  chisel. 

Finishing  the  Slides. — The  appearance  of  a  collection 
of  slides  depends  very  much  upon  the  style  in  which  they  are 
finished,  and  although  in  some  instances  it  may  be  said  that 
the  finish  does  not  affect  the  intrinsic  value  of  the  object,  it 
is  generally  the  case  that  a  well-finished  slide  is  more  durable 
than  one  that  has  not  been  properly  completed.  The  old  sys- 
tem of  covering  the  slides  with  paper  is  now  entirely  obsolete, 
and  properly  so.  It  was  troublesome,  unsightly,  unless  in  pro- 
fessional hands,  and  not  very  durable.  Fortunately  slides 
with  ground  and  polished  edges  are  now  so  cheap  that  there  is 
no  occasion  to  resort  to  the  paper  cover.  Objects  mounted  on 
these  slides,  whether  in  cells  or  otherwise,  are  in  general  cov- 
ered with  round  covers,  which  are  adjusted  on  the  turn-table 
so  as  to  be  perfectly  central.  After  the  mount  has  been  com- 
pleted so  far  as  fastening  down  the  cover  is  concerned,  the 
edge  of  the  latter  is  finished  with  a  neat  coat  of  varnish.  This 
varnish  serves  to  do  something  more  than  merely  ornament  the 
slide;  it  secures  the  cover  in  its  place,  and  prevents  the  drying 
up  of  the  medium  used  for  mounting.  Even  in  the  case  of 
Qa,nada  balsam  it  is  of  use,  for  if  gqlcl  sijae  lie  used  as  tliQ  var- 


OF  THE   MICROSCOPE.  233 

nish,  it  prevents  the  evaporation  of  the  turpentine,  and  the 
ultimate  drying  and  cracking  of  the  balsam.  Where  glycerine 
jelly,  glycerine,  or  glycerine  and  gum  are  used,  it  becomes 
indispensable. 

The  process  employed  for  finishing  slides  in  this  way  is  as 
follows:  The  objects  having  been  mounted,  the  slides  are  laid 
away  until  the  balsam,  cement,  etc.,  have  been  hardened,  when 
all  superfluous  matters  of  this  kind  are  easily  removed  with  a 
small  chisel  made  out  of  a  brad-awl  ground  thin  and  sharp.  A 
small  chisel-pointed  piece  of  hard  wood,  and  a  little  water,  will 
remove  the  last  traces  of  balsam  or  varnish,  and  if  necessary  a 
final  cleaning  may  be  given  with  a  rag  moistened  with  alcohol. 
The  slide  is  then  placed  on  the  turn-table,  and  a  neat  ring  of 
varnish,  either  plain  or  colored,  is  run  around  the  edge.  The 
varnish  used  for  this  purpose  should  be  selected  according  to 
the  material  in  which  the  object  is  mounted.  Thus,  for  ob- 
jects in  glycerine,  glycerine  jelly,  or  gum,  the  best  coating  is 
shellac  varnish,  which  may  be  left  quite  transparent  and 
colored  with  some  of  the  aniline  colors.  Shellac  also  answers 
on  Canada  balsam,  wh^n  the  latter  has  become  hard,  but  gold 
size  is  better,  and  the  gold  size  may  either  be  colored  with 
the  ordinary  artists'  colors,  which  are  sold  in  tubes,  and 
which  give  an  opaque-colored  ring,  or  transparent  colors  may 
be  used. 

Labeling  the  Slides. — The  proper  labeling  of  slides  and 
material  is  a  most  important  matter.  All  bottles  should  bo 
labeled,  not  only  on  the  bottles  themselves,  but  on  the  corks 
or  stoppers,  and  the  slides  should  be  kept  labeled  or  numbered 
during  every  stage  of  their  progress.  Our  system  is  as  follows: 
Before  the  object  is  mounted  the  slide  is  labeled  on  the  under 
side  with  a  very  thin  gummed  label.  Numbering  with  tho 
writing  diamond  is  deferred  until  the  mount  is  completed,  be- 
cause, if  spoilt,  the  whole  may  be  thrown  into  a  jar  to  be  soaked 
off,  and  this  cannot  be  done  with  figures  written  or  scratche  1 
in  with  a  diamond.  As  soon  as  the  slide  is  finished  the  regular 
label  is  attached,  and  the  slide  numbered  with  a  writing  dia- 
mond. Of  this  number  a  record  is  kept,  so  that  even  if  the  label 
should  fall  off  or  get  soaked  off,  a  nevr  label  may  l>3  provided ; 


234  SELECTION   AND   USE 

and  unfortunately  it  sometimes  happens  that  labels  drop  off 
either  from  exposure  to  moisture  or  excessive  dryness. 

Ajf  a&gards  designs,  etc.,  for  labels,  the  variety  is  endless. 
Each  microscopist  will  probably  select  the  one  that  accords 
most  nearly  with  his  own  taste.  The  only  suggestion  that  we 
would  make  is  that  severe  simplicity  be  adopted  as  the  rule. 
Complicated  and  fancy  labels  look  well  at  first,  but  they  soon 
pall,  and  we  get  tired  of  them.  It  is  well  to  have  a  large  blank 
label  at  one  end,  on  which  memoranda  may  be  written,  such  as 
the  power  best  adapted  to  show  the  object;  whether  it  is  best 
seen  by  reflected,  transmitted,  or  polarized  light;  the  location 
of  interesting  points  as  determined  by  the  Maltwood  Finder, 
etc.,  etc.  An  important  point  also  is  the  direction  in  which  the 
reading  should  run — whether  across  or  lengthwise  of  the  slide. 
There  seems  to  be  no  rule  on  this  point,  and  it  would  be  well 
for  our  societies  to  discuss  this  subject,  and  establish  a  rule  for 
the  benefit  of  exchangers.  Nothing  is  more  aggravating  than  a 
lot  of  slides  labeled  in  different  ways.  The  direction  will  de- 
pend, of  course,  upon  the  kind  of  cabinet  used.  For  cabinets 
with  racks,  either  way  will  answer,  though  the  lengthwise  direc- 
tion is  somewhat  the  most  convenient.  For  the  old  style  Eng- 
lish cabinet,  in  which  the  objects  lie  flat  and  endwise  to  the  front 
of  the  drawers,  the  writing  should  run  across  the  slide.  For  our 
American  cabinets  with  drawers  having  spaces  divided  off  for 
each  slide,  the  writing  should  be  lengthwise  of  the  slide.  In 
short,  when  the  slide  lies  in  its  proper  position  in  the  drawer,  the 
lines  of  writing  should  run  parallel  with  the  front  edge  of  the 
drawer. 

Cabinets. — The  value,  either  for  work  or  for  exhibition,  of 
a  collection  of  microscopic  objects,  is  greatly  enhanced  when 
they  are  properly  arranged  and  easily  accessible.  Conse- 
quently every  microscopist  who  possesses  even  a  small  number 
of  slides,  should  provide  some  kind  of  a  case  or  cabinet  for 
keeping  them. 

The  simplest  form  of  case  is  one  with  racks,  and  these  are 
made  in  a  great  variety  of  styles,  forms,  and  sizes,  from  the 
simple  mailing  box,  holding  two  or  three  slides,  to  large  and 
expensive  cabinets.  The  common  box,  with  a  hinged  lid,  and 


OF   THE    MICROSCOPE.  235 

holding  twenty-five  objects,  answers  well  for  carrying  a  few 
objects  to  exhibitions  and  the  like.  The  chief  objections  to 
this  style  of  cabinet  consist  in  the  difficulty  of  lifting  out  any 
particular  slide  and  of  reading  the  names.  The  latter  difficulty 
may  be  obviated  by  writing  the  names  on  the  bottom  of  the 
box  on  a  line  with  the  slide;  the  first  objection  may  be  miti- 
gated by  placing  the  spaces  in  the  rack  as  far  apart  as  possible. 
These  rack  boxes  make  altogether  the  cheapest  cases,  and  when 
placed  in  cabinets  holding,  say,  a  dozen  boxes,  or  three  hun- 
dred slides,  they  form  a  very  economical  and  convenient  ar- 
rangement. The  cabinet,  or  outer  case,  should,  of  course,  be  so 
made  that  the  boxes  will  stand  on  end,  as  in  this  way  the  slides 
lie  flat — a  most  important  point. 

Cases  or  cabinets  of  shallow  drawers  in  .which  the  slides  lie 
flat,  with  the  labels  fully  exposed  to  view,  are,  however,  alto- 
gether the  best.  They  have  been  made  of  various  forms.  A 
cheap,  simple  case,  holding  about  six  dozen  slides,  may  be  had, 
in  which  the  drawers  do  not  slide  in  grooves,  but  lie  one  above 
the  other.  The  only  objection  to  this  plan  lies  in  the  fact  that 
all  the  drawers  must  be  taken  out  if  we  wish  to  get  at  the  lowest 
one;  but  where  there  are  not  more  than  a  dozen  drawers  this  is 
not  a  very  serious  difficulty,  and  the  compactness,  lightness  and 
cheapness  of  the  arrangement  make  it  quite  a  favorite. 

In  the  English  cabinets  the  slides  lie  with  their  ends  towards 
the  front  of  the  drawer,  so  that  the  motion  of  pulling  the  drawer 
out  or  pushing  it  in,  does  not  cause  the  slides  to  slip  over  each 
other.  This  is  a  very  excellent  arrangement,  and  one  which 
we  like  better  than  any  other.  In  the  American  cabinets  as 
hitherto  made,  the  slides  lie  with  the  long  edges  towards  the 
front  of  the  drawers,  and  are  prevented  from  slipping  over 
each  other  by  small  partitions  which  divide  the  rows  into 
spaces  one  inch  each  'in  width.  This  is,  theoretically,  the  most 
perfect  arrangement,  but  it  requires  a  little  more  room  than 
the  other.  The  American  cabinets  have;  however,  one  feature 
which  is  a  most  important  one:  The  spaces  in  which  the  slides 
rest  have  a  depression  under  the  ends  of  the  slides,  and  by 
pressing  on  the  end,  the  slide  is  lifted  so  that  it  is  very  easily 
grasped.  In  the  English  cabinets  this  feature  is  wanting,  and 
it  is  only  with  great  trouble  that  a  tightly-fitting  slide  can  b« 


236  SELECTION   AND   USE 

taken  from  its  place.  Fortunately,  any  drawer  which  is  deep 
enough,  may  have  this  device  applied  to  it  by  simply  glueing  a 
thin  strip  of  wood  or  pasteboard  on  the  bottom  of  the  drawer 
so  as  to  raise  up  the  part  on  which  the  slides  rest,  but  leaving 
a  space  of  about  three-quarters  of  an  inch  under  the  end  of  the 
slide,  into  which  the  latter  may  be  tipped. 

The  M altwood  Finder. — This  is  a  most  important  ac- 
cessory to  every  microscope,  as  it  not  only  facilitates  inter- 
change of  notes  between 
microscopists  living  at  a 
distance  from  each  other, 
but  it  enables  observers  to 


MALTWOOD'S  jH 
FINDER 


make  an  accurate  record  of       (<_ 
the  position  of  any"  object, 

and  thus  make  sure  of  its  ri    79 

identity  when  under  ex- 
amination at  different  times.  It  consists  of  a  glass  slip,  a  lit- 
tle wider  than  an  ordinary  slide,  upon  which  is  a  photograph 
occupying  a  space  1  by  1  inch,  as  shown  in  figure  79.  This 
space  is  divided  into  2,500  squares  (50  divisions  on  each  side) 
and  each  of  these  small  squares  contains  two  numbers,  one  of 
which  indicates  its  position  from  bottom  to  top,  while  the 
other  marks  its  position  from  right  to  left.  Thus  the  square 
which  lies  on  the  tenth  line  from  the  bottom,  and  the  fifteenth 
from  the  right-hand  side,  would  be  }g. 

The  method  of  using  the  Finder  is  as  follows:  Placing  on  tho 
stage  an  object  mounted  on  an  ordinary  slide,  with  its  lower 
edge  against  a  ledge  of  some  kind,  and  its  left-hand  edge  against 
a  stop  (the  stop  and  ledge  both  being  movable  as  regards  the 
stage),  we  bring  some  particular  spot  into  view.  Removing 
the  slide,  we  now  place  the  finder  in  its  place,  and  read  off  the 
two  numbers.  It  is  no\v  evident  that  if  at  any  future  time  we 
should  place  the  finder  against  the  movable  ledge  and  stop,  and 
bring  the  same  numbers  into  view,  then  on  removing  the  finder 
and  placing  the  slide  on  the  stage  and  against  the  ledge  and 
stop,  which,  of  course,  must  occupy  the  same  position  that 
they  did  when  the  finder  was  in  place,  the  precise  spot  origin- 
allv  under  examination  will  be  in  view,  We  can  therefore 


Otf   THE   MICROSCOPE.  237 

easily  register  the  location  of  any  object  of  interest,  and  so  be 
certain  of  finding  it  at  any  future  time. 

The  mechanical  stage,  or  the  ordinary  movable  glass  stage, 
described  at  page  110,  or  that  shown  in  Fig.  24  (page  111)  af- 
fords special  facilities  for  using  the  Maltwood  Finder.  But  un- 
less fitted  with  some  special  contrivance,  microscopes  which 
have  only  the  ordinary 
clips,  are  unsuitable  for 
this  purpose.  A  movable 
ledge  and  stop,  which 
may  be  used  with  the 
common  spring  clips,  is, 
however,  easily  devised 

and  made.    We  often  use  8o._8TOP  FOR  MALTWOOD  FIN-DEI-, 

a  simple    wooden    slide 

with  a  little  projection  on  the  edge  near  one  end.  The  wooden 
slide  must  be  thicker  than  the  glass  slide  which  carries  the 
object  to  be  registered,  as  it  is  necessary  that  the  springs  should 
hold  the  wooden  slide  firmly  in  place,  while  the  object-slide 
moves  freely  below  the  springs.  A  piece  of  vulcanite  or  hard 
rubber  about  the  eighth  of  an  inch  thick  and  the  size  of  the 
usual  3x1  glass  slide  answers  admirably.  It  should  have  a 
stop  fastened  to  one  end,  as  shown  in  figure  80,  where  a  is  the 
hard  rubber  slide,  and  b  is  a  small  piece  of  brass,  let  into  the 
end  and  screwed  fast. 

Microscopical  Misinterpretations.— The  observer 
who  uses  the  compound  microscope  labors  under  certain  dis- 
advantages which  do  not  affect  those  who  examine  large  ob- 
jects which  can  be  handled,  and  thus  subjected  to  the  scrutiny 
of  several  senses.  The  fallacies  to  which  the  microscopist  is 
liable  in  this  way  deserve  special  attention  and  special  precau- 
tions. We  have  already  (page  198)  alluded  to  certain  fallacies 
of  another  kind,  which  must  be  avoided  by  such  careful  and 
extensive  study  as  will  enable  us  to  recognize  foreign  matter 
when  we  see  it;  the  fallacies  now  under  consideration  can  only 
be  avoided  by  careful  study  of  the  laws  of  optics,  and  by  intro- 
ducing considerable  variety  of  methods  into  our  examinations. 
One  of  these  sources  of  fallacy  arises  from  the  liability  which 
most  persons  have  to  see  .objects  pseudoscopicctlly,  as  it  is  called 
— that  is  to  say.  hollows  appear  to  be  elevations,  and  elevations 
appear  to  be  hollows.  The  extent  to  which  this  tendency  ex- 
ists is  not  generally  recognized.  Taking  a  gold  coin,  on  which 
the  letters,  etc.,  were  known  to  be  raised,  we  placed  it  under 
the  microscope,  and  submitted  it  to  seven  intelligent  persons. 
Out  of  these,  five  declared  that  they  saw  the  letters  sunk  into 


238  SELECTION  AND  USE  OF  THE  MICEOSCOPE. 

the  metal;  twp  said  they  were  raised.  In  objects  too  small  to 
be  felt,  and  where  sections  cannot  be  made,  the  truth  may  be 
ascertained  by  watching  the  effect  of  raising  or  lowering  the 
object  glass  in  focussing. 

Another  fallacy  of  this  kind  has  led  to  the  belief  that  hairs 
and  many  similar  bodies  are  hollow.  Seen  under  the  micro- 
scope, a  hair  looks  just  as  if  it  were  a  tube — but  then,  so  does 
a  wire,  which  is  known  to  be  solid.  The  test  in  this  case  is  to 
make  a  cross  section  of  the  object. 

The  true  form  of  objects  may  frequently  be  determined  by 
studying  the  effect  of  light  and  shade  produced  by  sending 
the  light  across  them  in  different  directions.  This  is  most 
readily  effected  by  means  of  the  revolving  stage,  which,  for  this 
purpose,  should  have  very  accurate  rotation  in  the  optic  axis. 

The  most  singular  fallacies,  however,  are  those  arising  from 
certain  illusions  of  vision,  which  affect  every  one,  and  which  in 
ordinary  practice,  are  easily  corrected.  For  a  full  account  of 
these,  the  reader  is  referred  to  the  works  of  Helmholtz;  a  brief 
account  of  the  most  common  cases  is  given  in  the  Young  Scientist 
for  1881.  The  one  which  is  of  most  interest  to  microscopists  is 
the  famous  optical  illusion  of  Nachet,  of  which  a  figure  is  given 
below.  In  the  course  of  his  examination  of  the  markings  on  the 


Fig.  St.— NATCHET'S  OPTICAL  ILLUSION. 

P.  Angulatum,  M.  Nachet  found  that  if  a  series  of  round  black 
dots  be  arranged  on  a  white  ground,  as  in  Fig.  81,  the  dots, 
when  viewed  from  a  distance  of  twelve  to  twenty  inches,  will 
appear  to  be  hexagonal,  though  we  know  that  they  are  round. 


FINIS. 


Plate  L 


ROSS    MODEL. 
A»    mad«    by    Ross    Si  Co. 


Plate  II/ 


ACME  NO.  4. 
As  made  by  J.   W.   Queen  &   Co. 


Plate  III. 


NEW    BIOLOGICAL   STAND. 
As  made  by  W.  H,  Bulloch. 


Plate  IV. 


THE   HISTOLOGICAL   MICROSCOPE. 

As  made  by  Joseph   Zentmayer, 


Slate  \, 


THE    INVESTIGATOR    MICROSCOPE 
As  made  by   Bausch  &  tomb  Optical  Company. 


Plate  VI. 


ARMY  HOSPITAL  BINOCULAR  STAND. 
As  made  by  Joseph  Zentmayer. 


Plate  VII. 


CONCENTRIC  STAND. 
As  made  by  Bausch  &  Lomb  Optical  Co. 


Kate  VIII. 


INVERTED  MICROSCOPE  STAND. 
As  made  by  Bausch  &  Lomb  Optical  Co. 


JAMES  W.  QUEEN  &  CO., 

MICROSCOPE  MAKERS, 

924  Chestnut  St.,  Philadelphia. 

THE  ACME  NO.  3  MICROSCOPE,  For  finest  work 
in  Bacteriology,  etc.,  being  well  fitted  for  all  the  wants  of  physicians, 
professors  and  amateurs,  who  require  an  instrument  of  high  grade  and 
adapted  to  receive  any  accessories. 
With  3-5  inch  and  1-5  inch  objectives,  2  eye-pieces  (power  60  to 

700),  glass  slides  and  cover,  in  case $83  00 

With  same  outfit,  but  with  addition  of  the  1-15  inch  oil  immersion 

objective  and  substage  condenser  (power  50  to  1,600)      .    .    $150  00 

THh  ACME 
NO.  4  MICROSCOPE 

is  pre-eminently  the  practising  physician's 
microscope,  having  lenses  of  such  power 
and  quality  as  will  clearly  show  the  diag- 
nostic test  of  consumption  (Bac.  tuber- 
culosis); amply  efficient  for  urinary  anal- 
ysis, etc.;  is  also  well  suited  for  use  of 
teachers  in  high  schools,  pharmacists,  and 
others  who  require  an  instrument  of 
thorough  workmanship  to  which  the  or- 
dinary accessories  may  be  added  (as  po- 
larizing apparatus,  etc.). 

With  1  in.  and  1-5  in.  objectives,  two 
eye-pieces  (power  40  to  500),  glass 
slides  and  covers  in  case  ....  $55  00 

THE  ACME  NO.  .5  MICROSCOPE  is  an  instrument 
of  simple  but  thorough  construction,  with  good  lenses,  and  at  a  mini- 
mum cost,  admirably  adapted  for  physicians'  daily  use  in  urinalysis, 
etc.,  tor  laboratory  use  in  colleges,  for  schools,  for  botanical  work,  and 
the  study  ot  nature  generally. 
With  1  inch  and  1-5  inch  objectives,  one  eye-piece  (power  40  to 

360),  in  case ' $2800 

The  same  with  rack-and-pinion  motion     .    .         $35  00 


Send  for  our  Complete  Catalogue  of  Microscopes,  and 
sample  copy  of  the  MICROSCOPICAL  BULLETIN— a  bi-monthly 
mag  zine. 

Correspondence  invited.  We  are  always  glad  to  yive  any  infoi  ma 
tion  required,  or  suggestions  to  aid  in  the  choice  of  an  instrument 
suitable  for  its  intended  purpose. 


SIXTH  EDITION.  Greatly  Enlarged,  with  over  80  {lustrations  in  the  Ttxt 
*nd '6  full  page  Engravings,  printed  on  Heavy  Tint  Paper,  i  Vol.  \-zmo.,  240- 
/,  iges.  Neatly  Bound  in  Cloth,  Gilt  Title.  Price  $1.25. 

HOW  TO  USE  THE  MICROSCOPE. 

\   SIMPLE   AND   PRACTICAL   BOOK,    INTENDED   FOR  BEGINNERS. 

BY  JOHN  PHIN, 
Editor  of  "  The  American  Journal  of  Microscopy." 


CONDENSED  TABLE  OF  CONTENTS. 

THE  MICROSCOPE. — What  it  Is;  What  it  Does;  Different  Kinds  of  Microscopes; 
I  rinciples  of  its  Construct  on;  Names  of  the  Different  Parts. 

SIMPLE  MICROSCOPES. —Hand  Magnifiers;  Doublets;  Power  of  Two  or  More 
Lenses  When  U>ed  Together;  Stanhope  Lens;  Coddington  Lens;  Achromatic 
Doublets  and  Trip'ets  ;  Twenty-five  Cent  Microscopes — and  How  to  Make  Them  ; 
Penny  Microscopes,  to  Show  Eels  in  Paste  and  Vinegar. 

DISSECTING  MICROSCOPES. — Essentials  of  a  Good  Dissecting  Microscope. 

COMPOUND  MICROSCOPES. — Cheap  Fo  eign  Siands ;  The  Ross  Model;  The  Jackson 
Model;  The  Continental  Model;  The  New  American  Model;  Cheap  American 
Stands;  The  Binocular  Microscope;  The  Binocular  Eye-piece;  The  Inverted  Micro- 
scope; Lithological  Microscopes;  The  Aquarium  Microscope;  Microscopes  for 
Special  Purpo  es;  "Class"  Microscopes. 

OBJECTIVES. — Defects  of  Common  Lenses;  Spherical  Aberration:  Chromatic  do.  ; 
Corrected  Objectives;  Defining  Power ;  Achromatism;  Aberration  r  .  Form ;  Flatness 
of  Field:  Angular  Aperture;  Penetrating  Power  '.  Working  Distan  ..3;  Immiersion  and 
"Homogeneous"  Lenses;  D  plex  Fronts;  French  Triplets,  etc.,  ftc. 

TESTING  OBJECTIVES.  —  General  Rules;  Accepted  Standards — Diatoms,  Ruled 
Lines,  Artificial  Star;  Podura;  Nnbert's  Lines;  M  oiler's  Probe  Platte,  etc.,  etc. 

SELECTION  OF  A  MICROSCOPE — Must  be  Adapted  to  Requirements  and  Skill  of 
User;  Microscopes  for  Botany;  For  Physicians;  For  Students. 

ACCESSORY  APPARATUS. — Stage  Forceps;  Forceps  Carrier;  Plain  Slides;  Concave 
Slides^;  Watch-Glass  Holder;  Animalcule  Cage;  Zoophyte  Trough  ;  The  Weber  Slide; 
The  Cell-Trough;  The  Compressorium  ;  Gravity  Compressorium ;  Growing  Slides; 
Frog  Plate;  Table  ;  Double  Nose-piece. 

ILLUMINATION. — Sun-Light;  Artificial  Light — Candles,  Gas,  Lamps,  etc.,  etc. 

ILLUMINATION  OF  OPAQUE  OBJECTS. — Bulls- Eye  Condenser;  Side  Reflector;  The 
Lieberkuhn;  The  Parabolic  Reflector;  Vertical  Illuminators. 

ILLUMINATION  OF  TRANSPARENT  OBJECTS. — Direct  and  Reflected  Light;  Axial  or 
Central  Ligb  ;  Oblique  Light;  The  Achromatic  Condenser;  The  Webster  Condenser, 
and  How  to  Usei* ;  Wenham's  Reflex  Illuminator,  and  How  to  Use  it;  The  Wenham 
Prism;  The  "  Half-Button ; "  The  Woodward  Illuminator;  Tulles'  Illuminating 
Traverse  Lens;  The  Spot  Lens;  The  Parabolic  Illuminator;  Polarized  Light. 

How  TO  USE  THE  MICROSCOPE — General  Rules;  Hints  to  Beginners. 

How   TO    USE   OBJECTIVES   OF    LARGE  APERTURE. — Collar-Correction,  etc. 

CARE  OF  THE  MICROSCOPE. — Should  be  Kept  Covered;  Care  of  Objectives:  Pre- 
cautions tr  be  Used  when  Corrosive  Vapors  and  Liquids  are  Employed ;  To  Protect 
th  Objectives  from  Vapors  which  Corrode  Glass ;  Cleaning  the  Objectives;  Cleaning 
th  Brass  Work 

COLLECTING  OBTECTS. — Where  to  Find  Objects ;  What  to  Look  for;  How  to  Capture 
Them. 

THE  PREPARATION  AND  EXAMINATION  OF  OBJECTS. — Cutting  Thin  Sections  of 
Sor  Substances;  Valentine's  Knife;  Sections  of  Wood  and  Bone;  Improved  Section 
Cutter;  Sections  of  Rock;  Knives;  Scissors;  Needles;  Dissecting  Pans  and  Dishes; 
Dissecting  Microscopes;  Separation  of  Deposits  from  Liquids;  Preparing  Whole 
Insects;  Feet,  Eye>,  Tongues,  Wings,  etc  ,  of  Insects;  Use  of  Chemical  Tests;  Liquids 
for  Moistening  Object >;  Refractive  Powers  of  D'fferent  Liquids;  lod-Serum  ,  Artinei»«r 
.od-Serum;  Covers  for  Keeping  Out  Dust;  Errors  in  Microscopic  Observations. 

PRESERVATIVE  PROCESSES. — General  Principles ;  Preservative  Media. 

APPARATUS  FOR  MOUNTING  OBJECTS. — Slides;  Covers;  Cells;  Turn-Tables,  e'tu. 

CEMENTS  AND  VARNISHES. — General  Rules  for  Using. 

MOUNTING  OBJECTS.— Mounting  Transparent  Object  Dry;  in  Balsam;  in  Liquid* 
Whole  Insects;  How  to  Get  Rid  of  Air-Bubbles;  Mounting  Opaque  Objects. 

FINISHING  THE  SLIDES. — Cabinets:  Maltwood  Finder;  Microscopical  Fallacies. 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 

OVERDUE.      ITflO       IT  rr     „« 

\  J(,  j 


J 

i&  or     ««ailiy^It  tfliJi  Yf 

REE   &B9S&J 

JAN    6  J955 

JAN  4    1955 

LD  21-100m-7,'40  (6936s) 

M88211 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


